Expose OBJT_MGTDEVICE VM objects used for GEM/TTM with drm2 as an
explicit object type.

Reviewed by:	kib
MFC after:	1 week
Don't waste a page of KVA for the boot-time memory test on x86.  For amd64,
reuse the first page of the crashdumpmap as CMAP1/CADDR1.  For i386,
remove CMAP1/CADDR1 entirely and reuse CMAP3/CADDR3 for the memory test.

Reviewed by:	alc, peter
MFC after:	2 weeks
Correct assertion to assert that the existing device VM object uses the
same type rather than asserting in the case where we just created a new
VM object.

Reviewed by:	kib
Swap PA and VA so they are in the right registers...