Enable DSC for DSI, if specified in VBT.

This still lacks DSC aware get config implementation, and therefore
state checker will fail. Also mode valid is not there yet.

v5:
- add dsc get config call

v4:
- convert_rgb = true (Vandita)
- ignore max cdclock check (Vandita)
- rename pipe_config to crtc_state

v3:
- take compressed bpp into account

v2:
- Nuke conn_state->max_requested_bpc, it's not used on DSI

Bspec: 49263
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e0136299e03c582238523189f6951eeb08daed98.1575974743.git.jani.nikula@intel.com