i965/vec4: Replace debug_flag with debug_enabled.

i965/vec4: Replace debug_flag with debug_enabled.

backend_visitor now handles this, so we can delete the vec4_visitor
specific code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>