drm/kms: add a function that can add the mode for the output device without EDID

Add a function that can be used to add the default mode for the output device
without EDID.
It will add the default mode that meets with the requirements of given
hdisplay/vdisplay limit.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>