Makefile: Use common code for DM_VIDEO deprecation warning

Update the CONFIG_DM_VIDEO check to use the 'deprecated' function.

Tested with pxm2

Old message:

===================== WARNING ======================
This board does not use CONFIG_DM_VIDEO Please update
the board to use CONFIG_DM_VIDEO before the v2019.07 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================

New message:

===================== WARNING ======================
This board does not use CONFIG_DM_VIDEO (Driver Model
for video). Please update the board to use
CONFIG_DM_VIDEO before the v2019.07 release. Failure to
update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
==================================================

Signed-off-by: Simon Glass <sjg@chromium.org>