radv: Limit workgroup size to 1024.

Fixes a hang with geekbench.

The existence of RX 580 and NAVI10 results shows that the generations
before and after this do not have the issue. (They show up on the
website). So this is likely a GFX9 only issue.

This is not something weird like LDS size since none of the shaders
seem to use LDS.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3145>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3145>
(cherry picked from commit a9a3108b)