anv: Assume syncobj support

anv: Assume syncobj support

Sync object for i915 support has been in upstream Linux since 4.14 which
is 3.5 years old at this point and, as far as we can tell, it also
exists in all the ChromeOS kernels.  Assuming it allows us to drop some
of our more gnarly synchronization fall-back paths.

At the time of merge, ChromeOS was on the following kernels:

 - kernel 3.18: SKL
 - kernel 4.4: BYT, KBL, APL
 - Kernel 4.14: BDW, GLK

All of the pre-4.14 kernels have had syncobj support back-ported.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9777>