drm/i915: Add function to call a power well's sync_hw() hook

Add a function to call a power well's sync_hw() hook, instead of
open-coding the same, as a step towards making the low-level
power well internals (i915_power_well_ops/desc structs) hidden.

The cached-enable state should be always up-to-date, so update it
whenever sync_hw() is called.

No functional change.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220222165137.1004194-7-imre.deak@intel.com