GL_RGBA8I is a supported image unit format, add
its corresponding UAV format on D3D backend.

Bug: angleproject:1987
TEST=dEQP.GLES31/functional_image_load_store_2d_store_rgba8i
     dEQP.GLES31/functional_image_load_store_2d_load_store_rgba8i

Change-Id: Ib759f873c8629a644f85784a57ce141d3018e0c1
Reviewed-on: https://chromium-review.googlesource.com/c/1378888Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>