Remove old fallback for vblank wait via manual drm ioctl

This workaround code has just been incrementally carried forward since
Cogl was integrated with Clutter but really we have no idea when this
code path was ever tested. Since the work around is from before the time
of the current Cogl developers we don't know anything about the
circumstances which led to this extreme workaround instead of pushing to
fix a driver.

It seems pretty likely we can push to fix any drm based drivers so
we're removing the workaround.

https://bugzilla.gnome.org/show_bug.cgi?id=667009



Reviewed-by: Neil Roberts <neil@linux.intel.com>