This is done to make it possible to change some of the DXGI formats
of integer textures without affecting their mip generation function
associations.

BUG=angleproject:1244
TEST=angle_end2end_tests,
     dEQP-GLES3.functional.texture.mipmap.*generate* (all pass)

Change-Id: Ie83dd0e1883e9d8e267fbd4bf54b1e466fb0b210
Reviewed-on: https://chromium-review.googlesource.com/328963Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>