drm/i915/dsc: split out encoder specific parts from DSC compute params

Split out the DP specific parts, making it easier to add DSI specific
configuration. Also move the encoder specific parts towards the end, to
allow overriding generic configuration if needed. This also improves
clarity by making it clear the encoder independent configuration does
not depend on the encoder specific parts.

v2: Rebase

Cc: Manasi Navare <manasi.d.navare@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/20191104141439.26312-3-jani.nikula@intel.com