drm/i915/pps: rename intel_edp_backlight_* to intel_pps_backlight_*

Follow 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/9887e4e278ed9a20da064bbf1d0845e52b7c3b3d.1610127741.git.jani.nikula@intel.com