Add an argument to the x86 pmap_invalidate_cache_range() to request
forced invalidation of the cache range regardless of the presence of
self-snoop feature.  Some recent Intel GPUs in some modes are not
coherent, and dirty lines in CPU cache must be flushed before the
pages are transferred to GPU domain.

Reviewed by:	alc (previous version)
Tested by:	pho (amd64)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week