intr/x86: replace use of vector in interface with intsrc

Several x86 interrupt core functions were already operating on intsrc
structures. Now switch the remaining 3 to intsrc for consistency.

Swap the order of intr_add_handler()'s first two arguments. This
matches INTRNG and is more consistent with other functions in this
interface.

Differential Revision: https://reviews.freebsd.org/D35386
Reviewed by: imp, markj (previous version)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1126