ac/llvm: fix mixing non-uniform/uniform sampler/texture descriptors

ac/llvm: fix mixing non-uniform/uniform sampler/texture descriptors

Non-uniform descriptors are not loaded in NIR and instead the pointer is
passed to abi->load_sampler_desc. It can be the case that only the texture
or sampler uses this path.

For the descriptor which doesn't use the path, we would previously pass
NULL to abi->load_sampler_desc.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19198>