i965: Move fallback size assignment out of bufmgr

The bufmgr took a mandatory size argument, which would only be used if
the kernel size query failed, i.e. an older kernel. It didn't actually
check that the BO size was sufficient for use.

Pull the check out of the bufmgr, and actually check that the BO is
sufficiently-sized for our import one level up. This also resolves a
chicken/egg we have when importing bufers without explicit modifiers,
namely that we need the tiling mode to calculate the size, but we need
the BO imported to query the tiling mode.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>