Add a compat kmap_atomic_prot_pfn to do quick kernel map / unmaps of

PCI- or high memory.
This is substantially more efficient than drm_bo_kmap,
since the mapping only lives on a single processor.
Unmapping is done use kunmap_atomic(). Flushes only a single tlb() entry.

Add a support utility int drm_bo_pfn_prot() that returns the
pfn and desired page protection for a given bo offset.

This is all intended for relocations in bound TTMS or vram.
Mapping-accessing-unmapping must be atomic, either using preempt_xx() macros
or a spinlock.