drm/amd/display: Use new connector state when getting color depth

[Why]
The current state on the connector is queried when getting the max bpc
rather than the new state. This means that a new max bpc value can only
currently take effect on the commit *after* it changes.

The new state should be passed in instead.

[How]
Pass down the dm_state as drm state to where we do color depth lookup.

The passed in state can still be NULL when called from
amdgpu_dm_connector_mode_valid, so make sure that we have reasonable
defaults in place. That should probably be addressed at some point.

This change now (correctly) causes a modeset to occur when changing the
max bpc for a connector.

v2: Drop extra TODO.

Cc: Leo Li <sunpeng.li@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>