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>