texture_index is meaningless when a tex_instr has deref src.
Use var->data.binding instead.
This fixes the incorrect lowering on radeonsi where the same
lowering steps was applied to all tex_instr based on the needs
of the first one (since texture_index is always 0).
CC: mesa-stable
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9931>
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