CVE-2024-43903 drm/amd/display: Add NULL check for 'afb' before dereferencing in amdgpu_dm_plane_handle_cursor_update

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

drm/amd/display: Add NULL check for ‘afb’ before dereferencing in amdgpu_dm_plane_handle_cursor_update

This commit adds a null check for the ‘afb’ variable in the
amdgpu_dm_plane_handle_cursor_update function. Previously, ‘afb’ was
assumed to be null, but was used later in the code without a null check.
This could potentially lead to a null pointer dereference.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/…/display/amdgpu_dm/amdgpu_dm_plane.c:1298 amdgpu_dm_plane_handle_cursor_update() error: we previously assumed ‘afb’ could be null (see line 1252)