nir: return progress from nir_lower_packing

nir: return progress from nir_lower_packing

Compiling with clang warns about an unused variable in
nir_lower_packing.

Tracking progress was added to nir_lower_packing in
adb157ddfd52ab495878a8b2b1bf70586d773aa2 but the function
will ignore the progress from impl calls and always return
false.

This patch changes it to return the progress. It fixes the
warning and should enable validation calls in NIR_PASS when
progress is made.

Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Fixes: adb157ddfd52 "nir: Return progress from nir_lower_64bit_pack()"
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11615>