i965/sync: Fail sync creation when batchbuffer flush fails

i965/sync: Fail sync creation when batchbuffer flush fails

Pre-patch, brw_sync.c ignored the return value of
intel_batchbuffer_flush().

When intel_batchbuffer_flush() fails during eglCreateSync
(brw_dri_create_fence), we now give up, cleanup, and return NULL.

When it fails during glFenceSync, however, we blindly continue and hope
for the best because there does not exist yet a way to tell core GL that
sync creation failed.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Tested-by: Rafael Antognolli <rafael.antognolli@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>