compositor-drm: Record current DPMS level
compositor-drm: Record current DPMS level

Track the current DPMS level for any given output, and track failure (or
otherwise) of the DPMS-setting property call.

Signed-off-by: Daniel Stone <62e7e355d5d8400fa8384448796af0bf6be4ffae@collabora.com>
Reviewed-by: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>

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