drm/i915/dsi: add support for DSC

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