drm/i915/pps: add locked intel_pps_wait_power_cycle

Prefer keeping the unlocked variants hidden if possible. No functional
changes.

Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4b712770deab9de8c3aeea8df35269433977038a.1610127741.git.jani.nikula@intel.com