drm/i915: get power domain in case the BIOS enabled eDP VDD

If I unplug the eDP monitor, the BIOS of my machine will enable the
VDD bit, then when the driver loads it will think VDD is enabled. It
will detect that the eDP is not enabled and return false from
intel_edp_init_connector. This will trigger a call to
edp_panel_vdd_off_sync(), which trigger a WARN saying that the
refcount of the power domain is less than zero.

The problem happens because the driver gets a refcount whenever it
enables the VDD bit, and puts the refcount whenever it disables the
VDD bit. But on this case, the BIOS enabled VDD, so all we do is to
call put() without calling get() first, so the code added is there to
make sure we always have the get() in case the BIOS enabled the bit.

This regression was introduced in
commit e9cb81a228
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Thu Nov 21 13:47:23 2013 -0200

    drm/i915: get a runtime PM reference when the panel VDD is on

v2: - Rebase

Tested-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org (v3.13+)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -3619,7 +3619,8 @@ static bool intel_edp_init_connector(struct intel_dp *intel_dp,
 {
 	struct drm_connector *connector = &intel_connector->base;
 	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
-	struct drm_device *dev = intel_dig_port->base.base.dev;
+	struct intel_encoder *intel_encoder = &intel_dig_port->base;
+	struct drm_device *dev = intel_encoder->base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_display_mode *fixed_mode = NULL;
 	bool has_dpcd;
@@ -3629,6 +3630,14 @@ static bool intel_edp_init_connector(struct intel_dp *intel_dp,
 	if (!is_edp(intel_dp))
 		return true;
 
+	/* The VDD bit needs a power domain reference, so if the bit is already
+	 * enabled when we boot, grab this reference. */
+	if (edp_have_panel_vdd(intel_dp)) {
+		enum intel_display_power_domain power_domain;
+		power_domain = intel_display_port_power_domain(intel_encoder);
+		intel_display_power_get(dev_priv, power_domain);
+	}
+
 	/* Cache DPCD and EDID for edp. */
 	intel_edp_panel_vdd_on(intel_dp);
 	has_dpcd = intel_dp_get_dpcd(intel_dp);