drm/i915: Check for NULL atomic state in intel_crtc_disable_noatomic()

In intel_crtc_disable_noatomic(), bail on a failure to allocate an
atomic state to avoid a NULL pointer dereference.

Fixes: 4a806558 ("drm/i915: Pass atomic state to crtc enable/disable functions")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v4.9+
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484922525-6131-4-git-send-email-ander.conselvan.de.oliveira@intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -6882,6 +6882,12 @@ static void intel_crtc_disable_noatomic(struct drm_crtc *crtc)
 	}
 
 	state = drm_atomic_state_alloc(crtc->dev);
+	if (!state) {
+		DRM_DEBUG_KMS("failed to disable [CRTC:%d:%s], out of memory",
+			      crtc->base.id, crtc->name);
+		return;
+	}
+
 	state->acquire_ctx = crtc->dev->mode_config.acquire_ctx;
 
 	/* Everything's already locked, -EDEADLK can't happen. */