radv: fix GPU hangs with 32-bit indirect descriptors

LLVM 6 isn't affected.

Fixes GPU hangs with new CTS:
dEQP-VK.binding_model.descriptorset_random.*

CC: 18.2 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 063264db)