intel/isl: Set null surface format to R32_UINT

It appears we never had a test in piglit or deqp sampling from a null
surface...

It turns out this triggers a hang on IVB only. Updating the null
surface format to R32_UINT fixes the hang on ivb and doesn't affect
other platforms, so set it by default for all platforms.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Gitlab: https://gitlab.freedesktop.org/mesa/mesa/issues/1872


Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit c445d6f6)