drm/i915: Split out dg2_crtc_compute_clock()

DG2 doesn't currently used the shared_dpll stuff so let's just
split it out from hsw_crtc_compute_clock() entirely.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220325123205.22140-9-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>