drm: add drm_set_preferred_mode

New helper function to set the preferred video mode.  Can be called
after drm_add_modes_noedid if you don't want the largest supported
video mode be used by default.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>