i965: Delete type field from backend_reg.

i965: Delete type field from backend_reg.

Switching from an implicitly-sized type field to field with an explicit
bit width is safe because we have fewer than 2^4 types, and gcc will
warn if you attempt to set a value that will not fit.

Reviewed-by: Emil Velikov <emil.velikov@collabora.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>