xrandr: Port some checks to XCB so we don't have to deal with BadName

RandR's QueryOutputProperty request makes the incredible decision of
throwing a BadName if you pass a property that doesn't exist, which
means that trying to check if a property exists is a royal pain when
using Xlib.

XCB's interface is much more friendly about errors and not having global
state and things like that, so use that instead to query our backlight
property.