radeonsi: use slot indexes for bindless handles

radeonsi: use slot indexes for bindless handles

Using VRAM address as bindless handles is not a good idea because
we have to use LLVMIntToPTr and the LLVM CSE pass can't optimize
because it has no information about the pointer.

Instead, use slots indexes like the existing descriptors. Note
that we use fixed 16-dword slots for both samplers and images.
This doesn't really matter because no real apps use image handles.

This improves performance with DOW3 by +7%.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>