compositor-drm: Return the newly added mode in drm_output_add_mode()
compositor-drm: Return the newly added mode in drm_output_add_mode()

Most of the times the caller will look at the output's mode list to get
the new mode, so just return that instead.

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