drm: Take vt(4) default mode from loader tunables

By default, vt(4) gets the "preferred mode" from DRM, when using a DRM
video driver as its backend. The preferred mode is usually the native
screen resolution.

Now, if this mode isn't appropriate, a user can use loader tunables to
select a mode. The tunables are read in the following order:

1. kern.vt.fb.modes.$connector_name
2. kern.vt.fb.default_mode

For example, to set a 1024x768 mode, no matter the connector:

kern.vt.fb.default_mode="1024x768"

To set a 800x600 mode only on the laptop builtin screen:

kern.vt.fb.modes.LVDS-1="800x600"

MFC after: 1 week