Add a shader cap for specifying the preferred shader representation.
Right now the only supported value is TGSI, other enum values will be
added as they are needed.

This is mainly to accommodate AMD's LLVM compiler back-end by letting
it bypass the TGSI representation for compute programs.  Other drivers
will keep using the common TGSI instruction set.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>