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_opregion.c b/drivers/gpu/drm/i915/intel_opregion.c
index 4d33874..42faa58 100644
--- a/drivers/gpu/drm/i915/intel_opregion.c
+++ b/drivers/gpu/drm/i915/intel_opregion.c
@@ -152,7 +152,6 @@
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct opregion_asle __iomem *asle = dev_priv->opregion.asle;
-	u32 max;
 
 	DRM_DEBUG_DRIVER("bclp = 0x%08x\n", bclp);
 
@@ -163,8 +162,7 @@
 	if (bclp > 255)
 		return ASLE_BACKLIGHT_FAILED;
 
-	max = intel_panel_get_max_backlight(dev);
-	intel_panel_set_backlight(dev, bclp * max / 255);
+	intel_panel_set_backlight(dev, bclp, 255);
 	iowrite32((bclp*0x64)/0xff | ASLE_CBLV_VALID, &asle->cblv);
 
 	return 0;