drm/i915: flip on a no fb -> fb transition if crtc is active v3

If the crtc is active, we can simply flip a new fb onto it, provided the
other mode setting reqs are met.  Otherwise, we'll need to do a full
mode set to re-enable the crtc.

v2: check for crtc active and set mode_changed accordingly
v3: add module parameter, i915.fastboot, to control no fb -> fb flip behavior
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -8676,8 +8676,16 @@ intel_set_config_compute_mode_changes(struct drm_mode_set *set,
 	} else if (set->crtc->fb != set->fb) {
 		/* If we have no fb then treat it as a full mode set */
 		if (set->crtc->fb == NULL) {
-			DRM_DEBUG_KMS("crtc has no fb, full mode set\n");
-			config->mode_changed = true;
+			struct intel_crtc *intel_crtc =
+				to_intel_crtc(set->crtc);
+
+			if (intel_crtc->active && i915_fastboot) {
+				DRM_DEBUG_KMS("crtc has no fb, will flip\n");
+				config->fb_changed = true;
+			} else {
+				DRM_DEBUG_KMS("inactive crtc, full mode set\n");
+				config->mode_changed = true;
+			}
 		} else if (set->fb == NULL) {
 			config->mode_changed = true;
 		} else if (set->fb->pixel_format !=