drm/i915/skl: Do not allow scaling when crtc is disabled.

This fixes a warning when the crtc is turned off. In that case fb
will be NULL, and crtc_clock will be 0. Because the crtc is no longer
active this is not a bug, and shouldn't trigger the WARN_ON.

Also remove handling a null crtc_state, with all transitional helpers
gone this can no longer happen.
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448360945-5723-2-git-send-email-maarten.lankhorst@linux.intel.comReviewed-by: NMika Kahola <mika.kahola@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -13850,7 +13850,7 @@ skl_max_scale(struct intel_crtc *intel_crtc, struct intel_crtc_state *crtc_state
 	struct drm_i915_private *dev_priv;
 	int crtc_clock, cdclk;
 
-	if (!intel_crtc || !crtc_state)
+	if (!intel_crtc || !crtc_state->base.enable)
 		return DRM_PLANE_HELPER_NO_SCALING;
 
 	dev = intel_crtc->base.dev;