drm/i915/backlight: extract backlight code to a separate file

In a long overdue refactoring, split out backlight code to new
intel_backlight.[ch]. Simple code movement, leave renames for follow-up
work. No functional changes.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/97d310848f03061473b9b2328e2c5c4dcf263cfa.1629888677.git.jani.nikula@intel.com

drivers/gpu/drm/i915/Makefile

@@ -249,6 +249,7 @@ i915-y += \
 	display/g4x_dp.o \
 	display/g4x_hdmi.o \
 	display/icl_dsi.o \
+	display/intel_backlight.o \
 	display/intel_crt.o \
 	display/intel_ddi.o \
 	display/intel_ddi_buf_trans.o \

drivers/gpu/drm/i915/display/g4x_dp.c

@@ -7,6 +7,7 @@
 
 #include "g4x_dp.h"
 #include "intel_audio.h"
+#include "intel_backlight.h"
 #include "intel_connector.h"
 #include "intel_de.h"
 #include "intel_display_types.h"
@@ -16,7 +17,6 @@
 #include "intel_fifo_underrun.h"
 #include "intel_hdmi.h"
 #include "intel_hotplug.h"
-#include "intel_panel.h"
 #include "intel_pps.h"
 #include "intel_sideband.h"
 

drivers/gpu/drm/i915/display/icl_dsi.c

@@ -29,6 +29,7 @@
 #include <drm/drm_mipi_dsi.h>
 
 #include "intel_atomic.h"
+#include "intel_backlight.h"
 #include "intel_combo_phy.h"
 #include "intel_connector.h"
 #include "intel_crtc.h"