pan/bi: Fix ms_idx type to catch missing ms_index source

nir_tex_instr_src_index() returns a negative result when the requested
source does not exist, but we cast that to an unsigned type thus losing
this information.

Fixes: b83c2936 ("pan/bi: Add basic support for txf_ms")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <!7314>