Add function for retrieving the DSC data for an encoder.

Initially, this is DSI specific, as DP does not use VBT settings for DSC
at all. It's also not very pretty.

In the future we might have a pointer from encoder to the child device,
which would make the child device list query here so much more sensible.

v3:
- use crtc_state instead of pipe_config
- return true by default from intel_bios_get_dsc_params()
- expand the comment about rc_buffer_block_size and rc_buffer_size

v2:
- make more robust, debug log errors better

Bspec: 29885
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/b895c349d964d70e4cad26f12a629ea1898bfcc2.1575974743.git.jani.nikula@intel.com