compositor: Display option help text only for enabled backends
compositor: Display option help text only for enabled backends

Signed-off-by: Bryce Harrington <bdb14cfc1ab8d24e13de84adc40b41ae60c13bd1@osg.samsung.com>
Reviewed-by: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>

sir-murray/weston,jonnylamb/weston,Fantu/compositor-spice,Fantu/compositor-spice,xorgy/weston,mchalupa/weston,krezovic/weston,giucam/weston,Gnurou/weston,udoprog/weston,eyolfson/weston,jonnylamb/weston,mchalupa/weston,Gnurou/weston,krezovic/weston,giucam/weston,Tarnyko/weston-xdg_surface_present,eyolfson/weston,udoprog/weston,kwm81/weston,sir-murray/weston,Tarnyko/weston-xdg_surface_present,kwm81/weston,Fantu/compositor-spice,xorgy/weston