Do not use fence sync on the i.MX6

The change to check for the correct EGL_KHR_fence_sync extension
revealed that on the i.MX6 SabreLite the eglClientWaitSyncKHR call
results in a crash, but since for this device glFlush() seems to
be enough to sync GL commands between threads, we can skip creating
a fence if we run on this hardware.

Change-Id: I208c7f390c9ceb792720f4e060c4a76c5db53e8f
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>