i965/vec4: Wrap vec4_generator in a C function.

i965/vec4: Wrap vec4_generator in a C function.

vec4_generator is a class for convenience, but only exports a single
method as its public API.  It makes much more sense to just export a
single function.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>