Fix mipmap generation on Nexus 5x & 6p devices.

Just like copyImage, these want the Z range of blit regions to be [0,
1], even for 2D resources where Z should be ignored.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2039913007

Review-Url: https://codereview.chromium.org/2039913007
1 file changed