drm/i915/pps: rename pps_{,un}lock -> intel_pps_{,un}lock

Start following the usual naming pattern for functions. 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/e37623750c592c08720f3b340cf85862d0f0ca12.1610127741.git.jani.nikula@intel.com