drm/amdgpu: fix documentation around busy_percentage

Add rename the gpu busy percentage for consistency and
add the mem busy percentage documentation.

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