drm/i915/ipc: register debugfs only if IPC available

It looks like trying to enable IPC via debugfs on platforms that don't
have IPC resulted in dmesg info message about IPC being enabled, which
is clearly not possible and would not happen.

Seems sensible to register IPC debugfs only on platforms that have IPC.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b18edb4f96c9d2ec728ef04e6f99d161fe5641d1.1662983005.git.jani.nikula@intel.com