drm/amdgpu: add new sysfs interface for memory realtime utilization

A new sysfs interface mem_busy_percent is added for telling
how busy the VRAM is(in percentage).

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>