drm/amdgpu: be explicit about cpu vram access for driver BOs (v2)

For kernel driver BOs, be explicit about whether we need
vram access up front.  This avoids unecessary migrations and
avoids using visible vram for buffers were it's not needed.

v2: line wrap fixes

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>