drm/i915/pps: abstract intel_pps_vdd_off_sync

Add a locked version of intel_pps_vdd_off_sync_unlocked() that does
everything the callers expect it to. No functional changes.

v2: Fix typo (Anshuman)

Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1e722290208d827c5cae107fe41dbfe41a494793.1610127741.git.jani.nikula@intel.com