lavapipe: resolve border-color when creating sampler

lavapipe: resolve border-color when creating sampler

This is going to make it easier to implement the custom border-color
extension.

While we're moving the code, tweak the memset code a bit, so we don't do
any float-ism in the int-case. It doesn't change anything functionally,
just makes it slightly clearer what's going on here.

Reviewed-by: Joshua Ashton <joshua@froggi.es>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10320>