x86/intr: allow mutex recursion in intr_remove_handler

This is needed so interrupt handlers can be removed while the PIC is
resuming, it was previously not possible due to intr_resume holding the
intr_table_lock and intr_remove_handler recursing on it.

Sponsored by: Citrix Systems R&D
Reviewed by: kib (previous version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D4114