drm/amd/amdgpu: add sw_fini interface for df_funcs

add sw_fini interface of df_funcs.
This interface will remove sysfs file of df_cntr_avail
function.

The old behavior only create sysfs of df_cntr_avail
in sw_init, but never remove it for lack of sw_fini
interface. With this,driver will report create
sysfs fail when it's loaded for the second time.

Signed-off-by: Jack Zhang <Jack.Zhang1@amd.com>
Reviewed-by: Jonathan Kim <Jonathan.Kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>