i965: Make the constant surface interface take a normal byte size.

i965: Make the constant surface interface take a normal byte size.

This puts the rounding-up logic into the function itself instead of all
the callers having to manage it.  Also drop an "unused" comment in gen4,
as the stride *is* used for texbos (and will be for uniforms soon).

NOTE: This is a candidate for the 9.1 branch.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>