drm/fb: Fix pointer dereference before null check.

fb_crtc is used before a null check, so move the use after the null check.

This was just identified by inspection. I haven't actually observed a crash
here, so it is possible that the null check could be unnecessary.
Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1501696813-8807-1-git-send-email-david@lechnology.com

drivers/gpu/drm/drm_fb_helper.c

@@ -2410,9 +2410,9 @@ static void drm_setup_crtcs(struct drm_fb_helper *fb_helper,
 		struct drm_display_mode *mode = modes[i];
 		struct drm_fb_helper_crtc *fb_crtc = crtcs[i];
 		struct drm_fb_offset *offset = &offsets[i];
-		struct drm_mode_set *modeset = &fb_crtc->mode_set;
 
 		if (mode && fb_crtc) {
+			struct drm_mode_set *modeset = &fb_crtc->mode_set;
 			struct drm_connector *connector =
 				fb_helper->connector_info[i]->connector;