drm/i915: fix possible memory leak in intel_hdcp_auth_downstream()

'ksv_fifo' is malloced in intel_hdcp_auth_downstream() and should be
freed before leaving from the error handling cases, otherwise it will
cause memory leak.

Fixes: f26ae6a6 ("drm/i915: SRM revocation check for HDCP1.4 and 2.2")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190704104534.12508-1-weiyongjun1@huawei.com