radv: fix stencil only clears.

radv: fix stencil only clears.

If we are clearing stencil only, we still need to provide a
a valid Z output from the vertex shader, we can't rely
on the depth clear value having any meaning, as we use this
for the position output, and it could get clipped, so we
don't end up clearing anything.

Fixes:
dEQP-VK.renderpass.simple.stencil
since I added S8 support.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 3c730639740f9b1243e95d06e6608cb54649be9a)