drm/panthor: Fix handling of partial GPU mapping of BOs

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

drm/panthor: Fix handling of partial GPU mapping of BOs

This commit fixes the bug in the handling of partial mapping of the
buffer objects to the GPU, which caused kernel warnings.

Panthor didn't correctly handle the case where the partial mapping
spanned multiple scatterlists and the mapping offset didn't point
to the 1st page of starting scatterlist. The offset variable was
not cleared after reaching the starting scatterlist.

Following warning messages were seen.
WARNING: CPU: 1 PID: 650 at drivers/iommu/io-pgtable-arm.c:659 __arm_lpae_unmap+0x254/0x5a0
<snip>
pc : __arm_lpae_unmap+0x254/0x5a0
lr : __arm_lpae_unmap+0x2cc/0x5a0
<snip>
Call trace:
__arm_lpae_unmap+0x254/0x5a0
__arm_lpae_unmap+0x108/0x5a0
__arm_lpae_unmap+0x108/0x5a0
__arm_lpae_unmap+0x108/0x5a0
arm_lpae_unmap_pages+0x80/0xa0
panthor_vm_unmap_pages+0xac/0x1c8 [panthor]
panthor_gpuva_sm_step_unmap+0x4c/0xc8 [panthor]
op_unmap_cb.isra.23.constprop.30+0x54/0x80
__drm_gpuvm_sm_unmap+0x184/0x1c8
drm_gpuvm_sm_unmap+0x40/0x60
panthor_vm_exec_op+0xa8/0x120 [panthor]
panthor_vm_bind_exec_sync_op+0xc4/0xe8 [panthor]
panthor_ioctl_vm_bind+0x10c/0x170 [panthor]
drm_ioctl_kernel+0xbc/0x138
drm_ioctl+0x210/0x4b0
__arm64_sys_ioctl+0xb0/0xf8
invoke_syscall+0x4c/0x110
el0_svc_common.constprop.1+0x98/0xf8
do_el0_svc+0x24/0x38
el0_svc+0x34/0xc8
el0t_64_sync_handler+0xa0/0xc8
el0t_64_sync+0x174/0x178
<snip>
panthor : [drm] drm_WARN_ON(unmapped_sz != pgsize * pgcount)
WARNING: CPU: 1 PID: 650 at drivers/gpu/drm/panthor/panthor_mmu.c:922 panthor_vm_unmap_pages+0x124/0x1c8 [panthor]
<snip>
pc : panthor_vm_unmap_pages+0x124/0x1c8 [panthor]
lr : panthor_vm_unmap_pages+0x124/0x1c8 [panthor]
<snip>
panthor : [drm] *ERROR* failed to unmap range ffffa388f000-ffffa3890000 (requested range ffffa388c000-ffffa3890000)