drm/i915: Move the DPLL extra power domain handling up one level

drm/i915: Move the DPLL extra power domain handling up one level

The extra DPLL power domain is currently handled in three places:
- combo_pll_enable()
- combo_pll_disable()
- readout_dpll_hw_state()

First two of those are low level PLL funcs, but the third is a higher
level thing. So the current situation is rather inconsistent. Unify
this by moving the PLL enable/disable up one level. This also means
the extra power domain could be trivially be used by other platforms
as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231012123522.26045-4-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>