drm/i915/bios: fill in DSC rc_model_size from VBT

The VBT fields match the DPCD data, so use the same helper.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/43fba75d89525413aed0bdbb082c26b09458bd46.1607429866.git.jani.nikula@intel.com