drm/i915: invert the verbosity of intel_enable_fbc

We currently print a DRM_DEBUG_KMS message on the happy path and don't
print anything on the "failed to allocate" path. On some desktop
environments (e.g., Unity) I see the "scheduling delayed FBC enable"
thousands and thousands of times on my dmesg.

So kill the useless message for the happy case, saving a lot of dmesg
space, and properly signal the "kzalloc fail" case.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Zoltan Nyul <zoltan.nyul@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -381,6 +381,7 @@ void intel_enable_fbc(struct drm_crtc *crtc, unsigned long interval)
 
 	work = kzalloc(sizeof *work, GFP_KERNEL);
 	if (work == NULL) {
+		DRM_ERROR("Failed to allocate FBC work structure\n");
 		dev_priv->display.enable_fbc(crtc, interval);
 		return;
 	}
@@ -392,8 +393,6 @@ void intel_enable_fbc(struct drm_crtc *crtc, unsigned long interval)
 
 	dev_priv->fbc_work = work;
 
-	DRM_DEBUG_KMS("scheduling delayed FBC enable\n");
-
 	/* Delay the actual enabling to let pageflipping cease and the
 	 * display to settle before starting the compression. Note that
 	 * this delay also serves a second purpose: it allows for a