It is to optimize memory mapping latency, and also aviod
a page fault in a corner case of changing valid PDE into
PTE.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>