drm/i915: Extract _intel_{enable,disable}_shared_dpll()

We have a bit of duplicated code around the DPLL disabling. Extract
that to a new function, and for symmetry also do the same for the
enable direction.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231012123522.26045-5-ville.syrjala@linux.intel.com


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