drm: fix build error when SYSRQ is disabled

Fix build error when CONFIG_MAGIC_SYSRQ is not enabled:

drivers/gpu/drm/drm_fb_helper.c:915: error: 'sysrq_drm_fb_helper_restore_op' undeclared (first use in this function)
drivers/gpu/drm/drm_fb_helper.c:929: error: 'sysrq_drm_fb_helper_restore_op' undeclared (first use in this function)
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fb_helper.c

@@ -283,6 +283,8 @@ static struct sysrq_key_op sysrq_drm_fb_helper_restore_op = {
 	.help_msg = "force-fb(V)",
 	.action_msg = "Restore framebuffer console",
 };
+#else
+static struct sysrq_key_op sysrq_drm_fb_helper_restore_op = { };
 #endif
 
 static void drm_fb_helper_on(struct fb_info *info)