drm/i915: Prevent double dpms on

Arguably this is a bug in drm-core in that we should not be called twice
in succession with DPMS_ON, however this is still occuring and we see
FDI link training failures on the second call leading to the occassional
blank display. For the time being ignore the repeated call.

Original patch by Dave Airlie <airlied@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org

drivers/gpu/drm/i915/intel_display.c

@@ -2370,6 +2370,9 @@ static void intel_crtc_dpms(struct drm_crtc *crtc, int mode)
 	int pipe = intel_crtc->pipe;
 	bool enabled;
 
+	if (intel_crtc->dpms_mode == mode)
+		return;
+
 	intel_crtc->dpms_mode = mode;
 	intel_crtc->cursor_on = mode == DRM_MODE_DPMS_ON;
 
@@ -5164,7 +5167,7 @@ static void intel_crtc_init(struct drm_device *dev, int pipe)
 	dev_priv->pipe_to_crtc_mapping[intel_crtc->pipe] = &intel_crtc->base;
 
 	intel_crtc->cursor_addr = 0;
-	intel_crtc->dpms_mode = DRM_MODE_DPMS_OFF;
+	intel_crtc->dpms_mode = -1;
 	drm_crtc_helper_add(&intel_crtc->base, &intel_helper_funcs);
 
 	intel_crtc->busy = false;