drm/modes: Add DRM_SIMPLE_MODE()

This adds a helper macro to specify modes that only contain info about
resolution.

v2: Actually set the width and height (Ilia Mirkin)

Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190210131039.52664-5-noralf@tronnes.org