drm/amd/display: Allow Z8 for multiplane configurations on DCN35

drm/amd/display: Allow Z8 for multiplane configurations on DCN35

[Why]
Power improvement over DCN314, but also addresses a functional issue
where plane_state remains uncleared on pipes that aren't actually
active.

[How]
Update the check to allow for zero streams to be treated as z8 allow.
Update the check to remove plane count on the active stream case.

Z8 will still be blocked based on stutter duration, which is likely to
be the case for most multi plane configurations.

Reviewed-by: Gabe Teeger <gabe.teeger@amd.com>
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>