drm/i915: s/dev_priv/i915/ in the shared_dpll code

Do a s/dev_priv/i915/ pass over the shared_dpll code to
get the variable names into sync with modern standards.

v2: Rebase

Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231003200620.11633-5-ville.syrjala@linux.intel.com