Turns out future DSI specific parameters aren't workable with the
approach of having the encoder specific functions in intel_vdsc.c. Make
intel_dsc_compute_params() a helper that does the encoder independent
parts, and have encoder code call it. Move intel_dsc_dp_compute_params()
to intel_dp.c as intel_dp_dsc_compute_params().

No functional changes.

v2: Rename pipe_config to crtc_state while at it.

Cc: Manasi Navare <manasi.d.navare@intel.com>
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/620688ec302f7f49cc539c6c1653bfaf6092fce0.1575974743.git.jani.nikula@intel.com