gallium: distinguish between shader IR in get_compute_param

gallium: distinguish between shader IR in get_compute_param

For radeonsi, native and TGSI use different compilers and this results
in different limits for different IR's.

The set we strictly need for radeonsi is only the MAX_BLOCK_SIZE
and MAX_THREADS_PER_BLOCK params, but I added a few others as shader
related that seemed like they would also typically depend on the
compiler.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Dave Airlie <airlied@redhat.com>