drm/amd/display: Refactor dc_state interface

[WHY?]
Part of the dc_state interface that deals with adding streams and planes should
remain public, while others that deal with internal status' and subvp should be
private to DC.

[HOW?]
Move and rename the public functions to dc_state.h and private functions to
dc_state_priv.h. Also add some additional functions for extracting subvp meta
data from the state.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>