freedreno/a6xx: Avoid zero-init fd6_emit

freedreno/a6xx: Avoid zero-init fd6_emit

A pretty significant amount of time spent in fd6_draw_vbo is calling
memset to zero init the on-stack struct.  And a big part of the size
of the struct is fd6_state, of which we only need to initialize
num_groups to zero.  This is worth a 15% improvement in drawoverhead
test 0 ("no state change").

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20572>