drm/edid: move displayid validation to it's own function.

We need to use this for validating modeline additions.

Change-Id: I55fe5a92c614cf949de906bea9a7c84d64450bb2
Signed-off-by: Dave Airlie <airlied@redhat.com>
Git-commit: c97291774c1b867b56c3d439ddaec9a965cf559e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>