Revert "drm/amd/display: Add NULL check for 'afb' before dereferencing in...

Revert "drm/amd/display: Add NULL check for 'afb' before dereferencing in amdgpu_dm_plane_handle_cursor_update"

[WHY]
This patch is a dupplicate implementation of 14bcf29b, which we
are reverting due to a regression with kms_plane_cursor IGT tests.

This reverts commit 38e6f715.

Reviewed-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Tested-by: George Zhang <George.zhang@amd.com>
Signed-off-by: Ivan Lipski <ivlipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>