drm/panthor: Fix kernel panic on partial unmap of a GPU VA region

In the Linux kernel, the following vulnerability has been resolved:

drm/panthor: Fix kernel panic on partial unmap of a GPU VA region

This commit address a kernel panic issue that can happen if Userspace
tries to partially unmap a GPU virtual region (aka drm_gpuva).
The VM_BIND interface allows partial unmapping of a BO.

Panthor driver pre-allocates memory for the new drm_gpuva structures
that would be needed for the map/unmap operation, done using drm_gpuvm
layer. It expected that only one new drm_gpuva would be needed on umap
but a partial unmap can require 2 new drm_gpuva and that's why it
ended up doing a NULL pointer dereference causing a kernel panic.

Following dump was seen when partial unmap was exercised.
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000078
Mem abort info:
ESR = 0x0000000096000046
EC = 0x25: DABT (current EL), IL = 32 bits
SET = 0, FnV = 0
EA = 0, S1PTW = 0
FSC = 0x06: level 2 translation fault
Data abort info:
ISV = 0, ISS = 0x00000046, ISS2 = 0x00000000
CM = 0, WnR = 1, TnD = 0, TagAccess = 0
GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
user pgtable: 4k pages, 48-bit VAs, pgdp=000000088a863000
[000000000000078] pgd=080000088a842003, p4d=080000088a842003, pud=0800000884bf5003, pmd=0000000000000000
Internal error: Oops: 0000000096000046 [#1] PREEMPT SMP
<snip>
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : panthor_gpuva_sm_step_remap+0xe4/0x330 [panthor]
lr : panthor_gpuva_sm_step_remap+0x6c/0x330 [panthor]
sp : ffff800085d43970
x29: ffff800085d43970 x28: ffff00080363e440 x27: ffff0008090c6000
x26: 0000000000000030 x25: ffff800085d439f8 x24: ffff00080d402000
x23: ffff800085d43b60 x22: ffff800085d439e0 x21: ffff00080abdb180
x20: 0000000000000000 x19: 0000000000000000 x18: 0000000000000010
x17: 6e656c202c303030 x16: 3666666666646466 x15: 393d61766f69202c
x14: 312d3d7361203a70 x13: 303030323d6e656c x12: ffff80008324bf58
x11: 0000000000000003 x10: 0000000000000002 x9 : ffff8000801a6a9c
x8 : ffff00080360b300 x7 : 0000000000000000 x6 : 000000088aa35fc7
x5 : fff1000080000000 x4 : ffff8000842ddd30 x3 : 0000000000000001
x2 : 0000000100000000 x1 : 0000000000000001 x0 : 0000000000000078
Call trace:
panthor_gpuva_sm_step_remap+0xe4/0x330 [panthor]
op_remap_cb.isra.22+0x50/0x80
__drm_gpuvm_sm_unmap+0x10c/0x1c8
drm_gpuvm_sm_unmap+0x40/0x60
panthor_vm_exec_op+0xb4/0x3d0 [panthor]
panthor_vm_bind_exec_sync_op+0x154/0x278 [panthor]
panthor_ioctl_vm_bind+0x160/0x4a0 [panthor]
drm_ioctl_kernel+0xbc/0x138
drm_ioctl+0x240/0x500
__arm64_sys_ioctl+0xb0/0xf8
invoke_syscall+0x4c/0x110
el0_svc_common.constprop.1+0x98/0xf8
do_el0_svc+0x24/0x38
el0_svc+0x40/0xf8
el0t_64_sync_handler+0xa0/0xc8
el0t_64_sync+0x174/0x178