drm: Constify topology id

Make the topology id const since we don't want to change it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313162054.16009-2-ville.syrjala@linux.intel.com


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>