panfrost: limit sample_shading to bifrost and later

We haven't wired this up in the Midgard compiler, so we can't expose
sample shading on Midgard GPUs. This all seems fixable, because the KILL
instruction can update the coverage without the kill-flag (yeah, a bit
confusing naming), but until someone puts in the time to wire up that,
let's just disable the functionality to avoid crashes.

Fixes: 6bba718027 ("panfrost: Advertise SAMPLE_SHADING")
Reviewed-by: Eric R. Smith <eric.smith@collabora.com>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35881>

src/gallium/drivers/panfrost/pan_screen.c

@@ -639,7 +639,7 @@ panfrost_init_screen_caps(struct panfrost_screen *screen)
    caps->depth_clip_disable = true;
    caps->mixed_framebuffer_sizes = true;
    caps->frontend_noop = true;
-   caps->sample_shading = true;
+   caps->sample_shading = dev->arch >= 6;
    caps->fragment_shader_derivatives = true;
    caps->framebuffer_no_attachment = true;
    caps->quads_follow_provoking_vertex_convention = true;