drm/amd/display: Allow commits with no planes active

[Why]
Many userspace applications (and IGT) seem to expect that most drivers
can keep a CRTC active and enabled if there are no primary or overlay
planes.

DC is setup to handle this but only in the case where there are
absolutely no planes on the CRTC - no primary, cursor, or overlay.

[How]
Add a check to reject commits that have cursor planes enabled and
nothing else on CRTCs since we can't handle that. The new helper
does_crtc_have_active_cursor is used for this.

In atomic commit tail, we need to let DC know that there are zero
planes enabled when doing stream updates to let it disable and blank
pipes as appropriate.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>