radeonsi: Use param export count from si_llvm_export_vs in si_shader_vs

radeonsi: Use param export count from si_llvm_export_vs in si_shader_vs

This eliminates the error prone logic in si_shader_vs recalculating this
value.

It also fixes TGSI_SEMANTIC_CLIPDIST outputs incorrectly not being
counted for VS exports. They need to be counted because they are passed
to the pixel shader as parameters as well.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91193
Reviewed-by: Marek Olšák <marek.olsak@amd.com>