drm/i915/pps: rename intel_edp_panel_* to intel_pps_*

Follow the usual naming pattern for functions. We don't need to repeat
"panel" here. No functional changes.

v2: Fix comment (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/b858271bd4d9c4a2ce15a13301d7bd9f7d121eb5.1610127741.git.jani.nikula@intel.com