drm/amd/display: Make DSC FGCG a DSC block level function

drm/amd/display: Make DSC FGCG a DSC block level function

[Why]
FGCG shouldn't be called at the DC resource level as part of DSC
creation because dc_create is intended for SW init, not HW init, and
register access is not guaranteed to work at this phase.

[How]
Add a set_fgcg function at the DSC interface level.

Existing ASIC can continue using the function in DC resource to retain
current compatibility but further development should favor calling
the function pointer during init_hw (if it exists).

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