Add glFlush workaround for instanced draws on Skylake

Bug: skia:
Change-Id: I1292cc423b4d3d7fc2a9035d3ad251ad7e00f0f6
Reviewed-on: https://skia-review.googlesource.com/121700
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
6 files changed