drm/i915: Fix CRT hotplug detect by checking really no channels attached

For CRT hotplug detect status, we have four test results as blue
channel only, green channel only, both blue and green channel, and
no channel attached. Origin code only marks both blue and green channel
case as connected, but ignore other possible connected states. This one
trys to detect CRT by checking no channel attached case instead.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>