drm/i915/pps: rename edp_panel_* to intel_pps_*_unlocked

Follow the usual naming pattern for functions, both for the prefix and
the _unlocked suffix for functions that expect the lock to be held when
calling. 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/d119605ba3d9c86647a524375de2d7e3d57a5676.1610127741.git.jani.nikula@intel.com