drm/amdgpu/virt: add function to check MMIO (v2)

MMIO space can be blocked on virtualised device. Add this
function to check if MMIO is blocked or not.

Todo: need a reliable method such like communation
with hypervisor.

v2:
 - add comments inline

Signed-off-by: pding <Pixel.Ding@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>