i965: Unvirtualize brw_create_constant_surface; delete Gen7+ variant.

i965: Unvirtualize brw_create_constant_surface; delete Gen7+ variant.

Now that brw_create_constant_surface uses a virtual function internally,
it doesn't need to be virtual itself.  We can delete the Gen7+ variant
and simplify things.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>