r600: increase number of ubos by one to 14

r600: increase number of ubos by one to 14

Ideally we'd support 16 (d3d11 requires 15, and mesa subtracts one for non-ubo
constants), but that's kind of impossible (it would be only doable if either
we'd somehow merge the mesa non-ubo constants with the driver constants, or
only use the driver constants with vtx fetch instead of through the kcache
mechanism - the latter probably wouldn't be too bad).
For now just do as the comment already said, place the gs ring (not really
a const buffer in any case) which is only ever referred to through vc fetch
clauses at index 16. Throw in a couple asserts for good measure to make sure
the hw limit isn't exceeded.

Tested-by: Konstantin Kharlamov <hi-angel@yandex.ru>
Reviewed-by: Dave Airlie <airlied@redhat.com>