aco/ngg: Refactor gs_alloc_req in preparation for NGG GS.

Previously, this function inferred the vertex and primitive counts
from the gs_tg_info shader argument, but in case of NGG GS, it will
need to be calculated in runtime.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <mesa/mesa!6964>