radeonsi: add support for viewport array (v3)

radeonsi: add support for viewport array (v3)

This isn't pretty and I'd suggest it the pm4 interface builder
could be tweaked to do this more efficently, but I'd need
guidance on how that would look.

This seems to pass the few piglit tests I threw at it.

v2: handle passing layer/viewport index to fragment shader.
fix crash in blit changes,
add support to io_get_unique_index for layer/viewport index
update docs.
v3: avoid looking up viewport index and layer in es (Marek).

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