drm/i915: Split out glk_plane_min_cdclk()

Split the glk+ stuff into it's own version of the .min_cdclk()
vfunc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210330162416.18616-1-ville.syrjala@linux.intel.com


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