kms_winsys: Add cogl_kms_renderer_get_gbm api

Applications might want to get the underlying gbm device so add a getter for it.

https://bugzilla.gnome.org/show_bug.cgi?id=730536
kms-winsys: Add api that tells cogl to ignore a crtc

An application might for whatever reason want to control a specific output
directly and have cogl only swap the other outputs if any. So add an api that
allows setting a crtc to be ignored.

https://bugzilla.gnome.org/show_bug.cgi?id=730536