st/mesa: replace _mesa_sysval_to_semantic table with function

st/mesa: replace _mesa_sysval_to_semantic table with function

Instead of using an array indexed by SYSTEM_VALUE_x, just use a
switch statement.  This fixes a regression caused by inserting new
SYSTEM_VALUE_ enums but not updating the mapping to TGSI semantics.

v2: fix a few switch statement mistakes for compute-related enums

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>