v3d: Use stvpmd for non-uniform offsets in GS

v3d: Use stvpmd for non-uniform offsets in GS

The offset for the VPM write for storing outputs from the geometry
shader isn’t necessarily uniform across all the lanes. This can happen
if some of the lanes don’t emit some of the vertices. In that case the
offset for the subsequent vertices will be different in each lane. In
that case we need to use the stvpmd instruction instead of stvpmv
because it will scatter the values out.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3150
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5621>