draw/gs: use mask to limit vertex emission.

draw/gs: use mask to limit vertex emission.

When executing for a single primitive, the mask has only one active
lane, however the vertex emit emits for all the lanes, pass in
the active mask and write the excess lanes to the overflow slot.

Fixes:
glsl-1.50-gs-max-output -scan 1 20

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5555>