r600: fix viewport clipping handling (v2)

r600: 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 have enough info to program VPORT_PROVOKE).

Fixes: arb_viewport_array-render-viewport-2 and some CTS tests.
v2: drop vport provoke write, drop initial state writing this
on evergreen, only program it on evergreen.

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