drm/i915: Enable VBT based BL control for DP

Currently, BXT_PP is hardcoded with value '0'.
It practically disabled eDP backlight on MRB (BXT) platform.

This patch will tell which BXT_PP registers (there are two set of
PP_CONTROL in the spec) to be used as defined in VBT (Video Bios Timing
table) and this will enabled eDP backlight controller on MRB (BXT)
platform.

v2:
- Remove unnecessary information in commit message.
- Assign vbt.backlight.controller to a backlight_controller variable and
return the variable value.
v3:
- Rebased to latest code base.
- updated commit title.

Signed-off-by: Mustamin B Mustaffa <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
(cherry picked from commit 73c0fcac97bf7f4a6a61b825b205d1cf127cfca7)
Signed-off-by: Rodrigo Vivi <[e-mail]>

drivers/gpu/drm/i915/intel_dp.c

···
 bxt_power_sequencer_idx(struct intel_dp *intel_dp)
 {
 	struct drm_i915_private *dev_priv = to_i915(intel_dp_to_dev(intel_dp));
+	int backlight_controller = dev_priv->vbt.backlight.controller;
 
 	lockdep_assert_held(&dev_priv->pps_mutex);
 
 	/* We should never land here with regular DP ports */
 	WARN_ON(!intel_dp_is_edp(intel_dp));
 
-	/*
-	 * TODO: BXT has 2 PPS instances. The correct port->PPS instance
-	 * mapping needs to be retrieved from VBT, for now just hard-code to
-	 * use instance #0 always.
-	 */
 	if (!intel_dp->pps_reset)
-		return 0;
+		return backlight_controller;
 
 	intel_dp->pps_reset = false;
 
···
 	 */
 	intel_dp_init_panel_power_sequencer_registers(intel_dp, false);
 
-	return 0;
+	return backlight_controller;
 }
 
 typedef bool (*vlv_pipe_check)(struct drm_i915_private *dev_priv,