drm/amdgpu: Add sysfs files for returning VRAM/GTT info v2

Add 6 files that return (in bytes):
The total amount of VRAM/visible VRAM/GTT
and the current total used VRAM/visible VRAM/GTT

v2: Split used and total into separate files

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