drm/i915: keep max backlight internal to intel_panel.c

In preparation of adding locking to backlight, make max backlight value
(the modulation frequency the PWM duty cycle value must not exceed)
internal to intel_panel.c.

Have intel_panel_set_backlight() accept a caller defined range for level,
and scale input to max backlight value internally.

Clean up intel_panel_get_max_backlight() and usage internally.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 66922f8..3595c82 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -553,8 +553,8 @@
 				    int fitting_mode,
 				    const struct drm_display_mode *mode,
 				    struct drm_display_mode *adjusted_mode);
-extern u32 intel_panel_get_max_backlight(struct drm_device *dev);
-extern void intel_panel_set_backlight(struct drm_device *dev, u32 level);
+extern void intel_panel_set_backlight(struct drm_device *dev,
+				      u32 level, u32 max);
 extern int intel_panel_setup_backlight(struct drm_connector *connector);
 extern void intel_panel_enable_backlight(struct drm_device *dev,
 					 enum pipe pipe);