drm/i915/dp: Fix compilation of intel_dp_hdcp_check_link

drivers/gpu/drm/i915/intel_dp.c: In function ‘intel_dp_hdcp_check_link’:
drivers/gpu/drm/i915/intel_dp.c:5191:26: error: ?: using integer constants in boolean context [-Werror=int-in-bool-context]
   return ret >= 0 ? -EIO : ret;

Fixes: 20f24d77 ("drm/i915: Implement HDCP for DisplayPort")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180118161025.22700-1-chris@chris-wilson.co.uk