Temporarily disable the use of 1GB page mappings by the direct map.  There are currently two problems with the use of 1GB page mappings by the direct map.  First, at least one device driver uses pmap_extract() rather than DMAP_TO_PHYS() to translate a direct map address to a physical address. Unfortunately, neither pmap_extract() nor pmap_kextract() yet support 1GB page mappings.  Second, pmap_bootstrap() needs to interrogate the MTRRs to ensure that a 1GB page mapping doesn't span two MTRRs of different types.
Temporarily disable the use of 1GB page mappings by the direct map.  There
are currently two problems with the use of 1GB page mappings by the direct
map.  First, at least one device driver uses pmap_extract() rather than
DMAP_TO_PHYS() to translate a direct map address to a physical address.
Unfortunately, neither pmap_extract() nor pmap_kextract() yet support 1GB
page mappings.  Second, pmap_bootstrap() needs to interrogate the MTRRs to
ensure that a 1GB page mapping doesn't span two MTRRs of different types.

Reported and tested by: Daniel O'Connor
MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase