drm/amdgpu: Change the way to determine framebuffer type

Determine FRAMEBUFFER_PUBLIC/PRIVATE only based host-accessibility,
not peer-accesssibility

Signed-off-by: Gang Ba <gaba@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>