lima/ppir: refactor texture code to simplify scheduler

lima/ppir: refactor texture code to simplify scheduler

The 'varying fetch' pp instruction deals only with coordinates, and
'texture fetch' deals only with the sampler index.
Previously it was not possible to clearly map ppir_op_load_coords and
ppir_op_load_texture to pp instructions as the source coordinates were
kept in the ppir_op_load_texture node, making this harder to maintain.
The refactor is made with the attempt to clearly map ppir_op_load_coords
to the 'varying fetch' and ppir_op_load_texture to the 'texture fetch'.
The coordinates are still temporarily kept in the ppir_op_load_texture
node as nir has both sampler and coordinates in a single instruction and
it is only possible to output one ppir node during emit. But now after
lowering, the sources are transferred to the (always) created
ppir_op_load_coords node, and it should be possible to directly map them
to their pp instructions from there onwards.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Qiang Yu <yuq825@gmail.com>