drm/i915: Fix memory leak in intel_hdcp auth

Static code analysis tool reported memory leak in
intel_hdcp_auth_downstream. Fixing the memory leak.

v2: Rebase, move free to a cleanup label(Jani)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180404225957.7457-1-radhakrishna.sripada@intel.com