drm/i915/dsc: make rc_model_size an encoder defined value

Move the initialization of the rc_model_size from the common code into
encoder code, allowing different encoders to specify the size according
to their needs. Keep using the hard coded value in the encoders for now
to make this a non-functional change.

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/6843c4f6958619f7389180aa92fded7b9fdbb4ba.1607429866.git.jani.nikula@intel.com