Revert "drm/amd/display: Add null check for 'afb' in amdgpu_dm_update_cursor"

[WHY]
This commit introduced a regression with kms_plane_cursor IGT
tests, causing AMD Asics to fail these tests.

This reverts commit ce66ffd9.

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>