drm/i915/display: split out DSC and DSS registers

Relatively few places need the DSC and DSS register definitions. Move
them to intel_vdsc_regs.h.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301151949.1591501-1-jani.nikula@intel.com