compositor-drm: use udev_device_get_property_value() helper
compositor-drm: use udev_device_get_property_value() helper

Udev provides a convenient helper. Use it instead of working with the
property-list directly.

Signed-off-by: David Herrmann <fb53b2ddb8d141e0cb39d7c67c2f81b6bc2eb0f7@googlemail.com>

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