This query would return either the RTV format, the DSV format or the
texture format depending on the render target. This made the code hard
to understand.

getDXGIFormat() calls are replaced by querying the ANGLE format, and
explicitly choosing which associated DXGI format to query info for
(RTV format or DSV format).

This refactoring makes changing some format associations for integer
texture formats easier in the future.

BUG=angleproject:1244
TEST=angle_end2end_tests,
     dEQP-GLES3.functional.fbo.blit.* (no regressions)

Change-Id: Ibe3c03fc6b7768af1a131d4df3909a1e20a71228
Reviewed-on: https://chromium-review.googlesource.com/329102Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>