drm: Add drm_atomic_set_mode_for_crtc

Add a new helper, to be used later for blob property management, that
sets the mode for a CRTC state, as well as updating the CRTC enable/active
state at the same time.

v2: Do not touch active/mode_changed in CRTC state. Document return
    value. Remove stray drm_atomic_set_mode_prop_for_crtc declaration.

v3: Remove i915 changes, and leave it directly bashing crtc_state->mode
    for the meantime.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Tested-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_crtc_helper.c

@@ -937,10 +937,11 @@ int drm_helper_crtc_mode_set(struct drm_crtc *crtc, struct drm_display_mode *mod
 			crtc_state->crtc = crtc;
 	}
 
-	crtc_state->enable = true;
 	crtc_state->planes_changed = true;
 	crtc_state->mode_changed = true;
-	drm_mode_copy(&crtc_state->mode, mode);
+	ret = drm_atomic_set_mode_for_crtc(crtc_state, mode);
+	if (ret)
+		goto out;
 	drm_mode_copy(&crtc_state->adjusted_mode, adjusted_mode);
 
 	if (crtc_funcs->atomic_check) {