intel/fs,vec4: Clean up a repeated pattern with SSBOs

intel/fs,vec4: Clean up a repeated pattern with SSBOs

Everywhere we handle SSBO intrinsics, we have exactly the same pattern
for computing the index so we may as well make a helper for it.  We also
add a get_nir_src_imm to vec4 and use it for SSBO offsets.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>