radv: Implement buffer stores with less than 4 components.

radv: Implement buffer stores with less than 4 components.

We started using it in the btoi paths for r32g32b32, and the LLVM IR
checker will complain about it because we end up with intrinsics with
the wrong type extension in the name.

Fixes: 593996bc02 ("radv: implement buffer to image operations for R32G32B32")
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>