D3D11: Use texture format, not SRV format, for mip gen test. Using the SRV format could lead to using R32F for D32F formats, which is incorrect. BUG=angleproject:1075 TEST=dEQP-GLES3.functional.fbo.blit.* Change-Id: Ic81d51b888897a7f7c79e2ba1a24ab69f18d981c Reviewed-on: https://chromium-review.googlesource.com/286770 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>