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

Requires enabling the vblank machinery for them.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/2030


Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org