drm/amd/display: Relax requirements for CRTCs to be enabled

[Why]
As long as we have at least one non-cursor plane enabled on a CRTC then
the CRTC itself can remain enabled.

This will allow for commits where there's an overlay plane enabled but
no primary plane enabled.

[How]
Remove existing primary plane fb != NULL checks and replace them with
the new does_crtc_have_active_plane helper.

This will be called from atomic check when validating the CRTC.

Since the primary plane state can now potentially be NULL we'll need
to guard for that when accessing it in some of the cursor logic.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Acked-by: Bhawanpreet Lakha &lt;Bhawanpreet <Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>