st/nir: Disable varying packing when doing transform feedback.

st/nir: Disable varying packing when doing transform feedback.

The varying packing would result in st_nir_assign_var_locations() picking
new driver_locations, despite the pipe_stream_output already being set up
for the old driver location.  This left the gallium driver with no way to
work back to what varying was referenced by pipe_stream_output.

Fixes these tests on V3D:
dEQP-GLES3.functional.transform_feedback.random.separate.points.3
dEQP-GLES3.functional.transform_feedback.random.separate.points.7
dEQP-GLES3.functional.transform_feedback.random.separate.points.9
dEQP-GLES3.functional.transform_feedback.random.separate.triangles.3
dEQP-GLES3.functional.transform_feedback.random.separate.triangles.8

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>