add libsync.h helper

Rather than cut/pasting these couple ioctl wrappers everywhere, just
stuff them as static-inline into a header.

This is probably mostly used from mesa, but some drivers, test apps, etc
may also want to use it from libdrm.

v2: handle EINTR, add sync_accumulate() based on #dri-devel discussion,
    etc

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>