drm: remove the EDID blob stored in the EDID property when it is disconnected

Now the EDID property will be updated when the corresponding EDID can be
obtained from the external display device. But after the external device
is plugged-out, the EDID property is not updated. In such case we still
get the corresponding EDID property although it is already detected as
disconnected.

https://bugs.freedesktop.org/show_bug.cgi?id=26743

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>