powerpc busdma: Use pmap_quick_enter_page()/pmap_quick_remove_page() to handle
bouncing of unmapped buffers. Also treat userspace buffers as unmapped, to
avoid borrowing the UVA for copies. This allows sync'ing userspace buffers
outside the context of the owning process, and sync'ing bounced maps in
non-sleepable contexts.

This change is equivalent to r286787 for x86.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D3989