When switching VAOs, if we switch to a VAO which has disabled
attributes, we could occasionally in some edge cases not have a buffer
initialized to render with. Fix this by re-checking the current
value (disabled) attributes every VAO switch.

Probably a regression caused by d28758de:
"D3D11: Re-enable updateVertexBuffer dirty bits."

BUG=angleproject:2156
BUG=angleproject:1156

Change-Id: Ic1795f914b9b4fa846241b0b4f9a8fe9c1183320
Reviewed-on: https://chromium-review.googlesource.com/669963Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>