Allows pinning of buffers in the non-CPU visible portion of
vram.

v2: incorporate Michel's comments.
v3: rebase on Michel's patch
v4: rebase on Michel's v2 patch
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>