drm/amd/display: Fix Z8 support configurations

[Why]
It's not supported in multi-display, but it is supported in 2nd eDP
screen only.

[How]
Remove multi display support, restrict number of planes for all
z-states support, but still allow Z8 if we're not using PWRSEQ0.

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>