CompressedTexSubImage would disallow 1x1 or 2x2 mips being set for any mip
level except 0.  This made it impossible to set the data in the lowest mip
levels.

BUG=angle:623

Change-Id: I623414ae4fd5d6b051579285160296b03fd36486
Reviewed-on: https://chromium-review.googlesource.com/197250Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>