drm/i915: Check num_pipes before initializing audio component

when num_pipes is zero, it indicates there is no display and HDMI
audio doesn't exist.

v2: Move the check from caller to callee for consistency.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Elaine Wang <elaine.wang@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482142746-21663-1-git-send-email-elaine.wang@intel.com