drm/amd/display: Add support for multiple overlay planes

[Why]
We only allowed 1 overlay plane. But now some ASICS can support multiple
overlay planes.

[How]
Use max_slave_planes as the number of overlays we can support.

Also since we cannot draw cursor over a video plane, we need to make
sure that we reject commits where the topmost plane is a video plane
(overlay only).

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>