i965/vec4: Correctly handle uniform sources in generate_tes_add_indirect_urb_offset

Fixes failure in the new piglit test
tes-patch-input-array-vec2-index-invalid-rd.shader_test.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 75666605)