radv: Limit workgroup size to 1024 on GFX9.

Bas Nieuwenhuizen requested to merge bnieuwenhuizen/mesa:workgroup-size into master

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).

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

CC: mesa-stable@lists.freedesktop.org

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