drm/i915/cdclk: Extract dg2_power_well_count()

drm/i915/cdclk: Extract dg2_power_well_count()

Extract the code to determine the DG2 pipe power well count
into a small helper. I'll have other uses for this later.

TODO: need to move this power well stuff out from the cdclk code...

v2: Don't lose the early return from intel_cdclk_pcode_pre_notify()
    (kernel test robot)
v3: Compare old vs. new, not old vs. old (Jani)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250926083950.24486-1-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>