drm/amd/display: Ensure valid event timestamp for cursor-only commits

Requires enabling the vblank machinery for them.

Bug: drm/amd#2030


Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>