pan/bi: Use texture index instead of sampler for message preloading

The VAR_TEX definition in ISA.xml only has a field for texture_index,
so trying to read sampler_index will return zero; read from
texture_index instead, and rename other fields for consistency.

The texture and sampler indices must be equal for VAR_TEX to be used,
so either name could be used for the field.

Fixes the wrong textures being used in Thief.

Closes: #6219
Fixes: eb1479bd ("pan/bi: Support message preloading")
Part-of: <!16255>