i965/vs: Don't hardcode DEBUG_VS in generic vec4 code.

i965/vs: Don't hardcode DEBUG_VS in generic vec4 code.

Since the vec4_visitor and vec4_generator classes are going to be
re-used for geometry shaders, we can't enable their debug
functionality based on (INTEL_DEBUG & DEBUG_VS) anymore.  Instead, add
a debug_flag boolean to these two classes, so that when they're
instantiated the caller can specify whether debug dumps are needed.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>