fbdev: Fix unregistering of framebuffers without device

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

fbdev: Fix unregistering of framebuffers without device

OF framebuffers do not have an underlying device in the Linux
device hierarchy. Do a regular unregister call instead of hot
unplugging such a non-existing device. Fixes a NULL dereference.
An example error message on ppc64le is shown below.

BUG: Kernel NULL pointer dereference on read at 0x00000060
Faulting instruction address: 0xc00000000080dfa4
Oops: Kernel access of bad area, sig: 11 [#1]
LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
[...]
CPU: 2 PID: 139 Comm: systemd-udevd Not tainted 5.17.0-ae085d7f9365 #1
NIP: c00000000080dfa4 LR: c00000000080df9c CTR: c000000000797430
REGS: c000000004132fe0 TRAP: 0300 Not tainted (5.17.0-ae085d7f9365)
MSR: 8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE> CR: 28228282 XER: 20000000
CFAR: c00000000000c80c DAR: 0000000000000060 DSISR: 40000000 IRQMASK: 0
GPR00: c00000000080df9c c000000004133280 c00000000169d200 0000000000000029
GPR04: 00000000ffffefff c000000004132f90 c000000004132f88 0000000000000000
GPR08: c0000000015658f8 c0000000015cd200 c0000000014f57d0 0000000048228283
GPR12: 0000000000000000 c00000003fffe300 0000000020000000 0000000000000000
GPR16: 0000000000000000 0000000113fc4a40 0000000000000005 0000000113fcfb80
GPR20: 000001000f7283b0 0000000000000000 c000000000e4a588 c000000000e4a5b0
GPR24: 0000000000000001 00000000000a0000 c008000000db0168 c0000000021f6ec0
GPR28: c0000000016d65a8 c000000004b36460 0000000000000000 c0000000016d64b0
NIP [c00000000080dfa4] do_remove_conflicting_framebuffers+0x184/0x1d0
[c000000004133280] [c00000000080df9c] do_remove_conflicting_framebuffers+0x17c/0x1d0 (unreliable)
[c000000004133350] [c00000000080e4d0] remove_conflicting_framebuffers+0x60/0x150
[c0000000041333a0] [c00000000080e6f4] remove_conflicting_pci_framebuffers+0x134/0x1b0
[c000000004133450] [c008000000e70438] drm_aperture_remove_conflicting_pci_framebuffers+0x90/0x100 [drm]
[c000000004133490] [c008000000da0ce4] bochs_pci_probe+0x6c/0xa64 [bochs]
[...]
[c000000004133db0] [c00000000002aaa0] system_call_exception+0x170/0x2d0
[c000000004133e10] [c00000000000c3cc] system_call_common+0xec/0x250

The bug [1] was introduced by commit 27599aacbaef ("fbdev: Hot-unplug
firmware fb devices on forced removal"). Most firmware framebuffers
have an underlying platform device, which can be hot-unplugged
before loading the native graphics driver. OF framebuffers do not
(yet) have that device. Fix the code by unregistering the framebuffer
as before without a hot unplug.

Tested with 5.17 on qemu ppc64le emulation.