i915: ensure that VGA plane is disabled

Some broken systems (like HP nc6120) in some cases, usually after LID
close/open, enable VGA plane, making display unusable (black screen on LVDS,
some strange mode on VGA output). We used to disable VGA plane only once at
startup. Now we also check, if VGA plane is still disabled while changing
mode, and fix that if something changed it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57434
Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -9152,6 +9152,23 @@ static void intel_sanitize_encoder(struct intel_encoder *encoder)
 	 * the crtc fixup. */
 }
 
+static void i915_redisable_vga(struct drm_device *dev)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+	u32 vga_reg;
+
+	if (HAS_PCH_SPLIT(dev))
+		vga_reg = CPU_VGACNTRL;
+	else
+		vga_reg = VGACNTRL;
+
+	if (I915_READ(vga_reg) != VGA_DISP_DISABLE) {
+		DRM_DEBUG_KMS("Something enabled VGA plane, disabling it\n");
+		I915_WRITE(vga_reg, VGA_DISP_DISABLE);
+		POSTING_READ(vga_reg);
+	}
+}
+
 /* Scan out the current hw modeset state, sanitizes it and maps it into the drm
  * and i915 state tracking structures. */
 void intel_modeset_setup_hw_state(struct drm_device *dev,
@@ -9260,6 +9277,8 @@ void intel_modeset_setup_hw_state(struct drm_device *dev,
 			intel_set_mode(&crtc->base, &crtc->base.mode,
 				       crtc->base.x, crtc->base.y, crtc->base.fb);
 		}
+
+		i915_redisable_vga(dev);
 	} else {
 		intel_modeset_update_staged_output_state(dev);
 	}