radeonsi: fix viewport clipping handling. (v2)

radeonsi: fix viewport clipping handling. (v2)

If oViewport is written, vertex reuse need to be turned off.
If oViewport is constant, vertex reuse is fine, and VPORT_PROVOKE_DISABLE
need to be set. (We don't know if oViewport is constant so we
skip this.)

Fixes: arb_viewport_array-render-viewport-2 and some CTS tests.
v2: drop writing to provoke disable, drop write in initial
state.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>