i965: Initialize all member variables of bblock_t on construction.

i965: Initialize all member variables of bblock_t on construction.

The bblock_t object relies on the memory allocator zeroing out its
contents before it's initialized, which is quite an unusual practice
in the C++ world because it ties objects to some specific allocation
scheme, and gives unpredictable results when an object is created with
a different allocator -- Stack allocation, array allocation, or
aggregation inside a different object are some of the useful
possibilities that come to my mind.  Initialize all fields from the
constructor and stop using the zeroing allocator.

v2: Use zero initialization for numeric types instead of default construction.

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