Adds a helper routine weston_output_inhibited_outputs() which returns a
mask of outputs that should inhibit screen idling.

Use this routine to check for inhibiting outputs for handling of idle
behaviors in core:  In sleep mode, only halt repainting outputs that
don't have valid inhibits.  Don't send these monitors DPMS off commands
either, if the system would otherwise be powering them down.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>

v5: Drop unused view variable