Disable mipmap pre-generation path

There is a bug in the mipmap pre-generation logic in use in
getDeferredTextureImageData. This can cause runaway memory leaks, so we
are disabling this path until we can investigate further.

BUG=669775

Change-Id: I2027f6f7994e089edd4f3452284e894752b31779
Reviewed-on: https://skia-review.googlesource.com/5357
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-on: https://skia-review.googlesource.com/7285