draw/gs: reduce the size of the gs output buffer

draw/gs: reduce the size of the gs output buffer

We used to overallocate the output buffer sometimes running out
of memory with applications rendering large geometries. The actual
maximum number of vertices out is simply the maximum number of
primitives in (number of gs invocations) multiplied by the maximum
number of output vertices per gs input primitive (i.e. gs invocation).

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>