drm: get cea video id code for a given display mode

This patch adds support for getting CEA Video ID Code for a given
display mode after matching with edid_cea_modes list. Its index in
the list added with one, gives the desired code.

This exported function will be used by hdmi drivers for composing
AVI info frame data.

Signed-off-by: Stephane Marchesin <marcheu@chromium.org>
Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>