drm/i915: Don't dereference fb when disabling primary plane

During driver init we may not have a valid framebuffer for the primary
plane even though the plane is enabled due to failed BIOS fb takeover.
This means we have to avoid dereferencing the fb in
.update_primary_plane() when disabling the plane.

The introduction of the primary plane rotation in

 commit d91a2cb8e5104233c02bbde539bd4ee455ec12ac
 Author: Sonika Jindal <sonika.jindal@intel.com>
 Date:   Fri Aug 22 14:06:04 2014 +0530

    drm/i915: Add 180 degree primary plane rotation support

caused a regression by trying to look up the pixel format before we can
be sure there's a valid fb available. This isn't entirely unsurprising
since the rotation patches originally predate the change to the primary
plane code that calls .update_primary_plane() also when disabling the
plane:

 commit fdd508a6
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Fri Aug 8 21:51:11 2014 +0300

    drm/i915: Call .update_primary_plane in intel_{enable,
    disable}_primary_hw_plane()

v2: Warn but don't blow up when trying to enable a plane w/o an fb (Chris)

Cc: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -2388,15 +2388,13 @@ static void i9xx_update_primary_plane(struct drm_crtc *crtc,
 	struct drm_device *dev = crtc->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
-	struct drm_i915_gem_object *obj = intel_fb_obj(fb);
+	struct drm_i915_gem_object *obj;
 	int plane = intel_crtc->plane;
 	unsigned long linear_offset;
 	u32 dspcntr;
 	u32 reg = DSPCNTR(plane);
 	int pixel_size;
 
-	pixel_size = drm_format_plane_cpp(fb->pixel_format, 0);
-
 	if (!intel_crtc->primary_enabled) {
 		I915_WRITE(reg, 0);
 		if (INTEL_INFO(dev)->gen >= 4)
@@ -2407,6 +2405,12 @@ static void i9xx_update_primary_plane(struct drm_crtc *crtc,
 		return;
 	}
 
+	obj = intel_fb_obj(fb);
+	if (WARN_ON(obj == NULL))
+		return;
+
+	pixel_size = drm_format_plane_cpp(fb->pixel_format, 0);
+
 	dspcntr = DISPPLANE_GAMMA_ENABLE;
 
 	dspcntr |= DISPLAY_PLANE_ENABLE;
@@ -2510,15 +2514,13 @@ static void ironlake_update_primary_plane(struct drm_crtc *crtc,
 	struct drm_device *dev = crtc->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
-	struct drm_i915_gem_object *obj = intel_fb_obj(fb);
+	struct drm_i915_gem_object *obj;
 	int plane = intel_crtc->plane;
 	unsigned long linear_offset;
 	u32 dspcntr;
 	u32 reg = DSPCNTR(plane);
 	int pixel_size;
 
-	pixel_size = drm_format_plane_cpp(fb->pixel_format, 0);
-
 	if (!intel_crtc->primary_enabled) {
 		I915_WRITE(reg, 0);
 		I915_WRITE(DSPSURF(plane), 0);
@@ -2526,6 +2528,12 @@ static void ironlake_update_primary_plane(struct drm_crtc *crtc,
 		return;
 	}
 
+	obj = intel_fb_obj(fb);
+	if (WARN_ON(obj == NULL))
+		return;
+
+	pixel_size = drm_format_plane_cpp(fb->pixel_format, 0);
+
 	dspcntr = DISPPLANE_GAMMA_ENABLE;
 
 	dspcntr |= DISPLAY_PLANE_ENABLE;