drm/i915: Parse LFP brightness control field in VBT

These fields in VBT indicates the PWM source which
is used and also the controller number.

v2 by Jani: check for out of bounds access, some renames, change default
type, etc.

v3 by Jani: s/INTEL_BACKLIGHT_CABC/INTEL_BACKLIGHT_DSI_DCS/

Signed-off-by: Deepak M <m.deepak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/eee2f7b683a081f006a7df1ddad9b20fbf53c48c.1461676337.git.jani.nikula@intel.com
diff --git a/drivers/gpu/drm/i915/intel_bios.c b/drivers/gpu/drm/i915/intel_bios.c
index dbbd591..8151811 100644
--- a/drivers/gpu/drm/i915/intel_bios.c
+++ b/drivers/gpu/drm/i915/intel_bios.c
@@ -318,6 +318,15 @@
 		return;
 	}
 
+	dev_priv->vbt.backlight.type = INTEL_BACKLIGHT_DISPLAY_DDI;
+	if (bdb->version >= 191 &&
+	    get_blocksize(backlight_data) >= sizeof(*backlight_data)) {
+		const struct bdb_lfp_backlight_control_method *method;
+
+		method = &backlight_data->backlight_control[panel_type];
+		dev_priv->vbt.backlight.type = method->type;
+	}
+
 	dev_priv->vbt.backlight.pwm_freq_hz = entry->pwm_freq_hz;
 	dev_priv->vbt.backlight.active_low_pwm = entry->active_low_pwm;
 	dev_priv->vbt.backlight.min_brightness = entry->min_brightness;