nir: Fix uninitialized 'progress' variable in nir_lower_system_values.

nir: Fix uninitialized 'progress' variable in nir_lower_system_values.

Commit 0a1adaf11d051b71b4c46aabee2e5342f2d6aef3 (nir: Report progress
from nir_lower_system_values().) introduced a bug caught by Valgrind:

==823== Conditional jump or move depends on uninitialised value(s)
==823==    at 0xB09020C: convert_block (nir_lower_system_values.c:68)
==823==    by 0xB079FB8: foreach_cf_node (nir.c:1310)
==823==    by 0xB07A0AF: nir_foreach_block (nir.c:1336)
==823==    by 0xB09026B: convert_impl (nir_lower_system_values.c:79)
...
==823==  Uninitialised value was created by a stack allocation
==823==    at 0xB090249: convert_impl (nir_lower_system_values.c:76)

which is trivially fixed by initializing progress.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>