D3D11: Get mip generation function from ANGLE format 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/328963 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>