drm/i915: don't scream into dmesg when a modeset fails

There are legit cases, e.g. when userspace asks for something
impossible. So tune it down to debug output like we do with all other
userspace-triggerable warnings.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66111#c5Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Rebased.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -8753,7 +8753,7 @@ static int intel_crtc_set_config(struct drm_mode_set *set)
 	}
 
 	if (ret) {
-		DRM_ERROR("failed to set mode on [CRTC:%d], err = %d\n",
+		DRM_DEBUG_KMS("failed to set mode on [CRTC:%d], err = %d\n",
 			      set->crtc->base.id, ret);
 fail:
 		intel_set_config_restore_state(dev, config);