drm/i915: Only disable HDCP when it's active

Instead of always trying to disable HDCP. Only run hdcp_disable when the
state is not UNDESIRED. This will catch cases where it's enabled and
also cases where enable failed and the state is left in DESIRED mode.

Note that things won't blow up if disable is attempted while already
disabled, it's just bad form.

Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180109185330.16853-1-seanpaul@chromium.org