Add current/available compute partitin mode sysfs node.

v2: make the sysfs node as IP independent one in amdgpu_gfx.c
Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>