radeonsi: fix a coherency issue when VS memory stores are not visible in PS

radeonsi: fix a coherency issue when VS memory stores are not visible in PS

If a shader has no param exports (no varyings), the pixel shader can start
after the VS position is written before the vertex shader finishes.
The fix is to wait for the memory stores before the position export.

The code needs to be restructured. First prepare param exports to get
nr_param_exports, then emit position exports with the wait, and then
emit param exports.

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10813>