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"

Beside r274031, this MFC includes:

r274049:
  drm: When reading connector mode tunables, list connectors

  ... and their associated tunables. This gives a way to know the list of
  available connectors, no matter the driver.

  The problem is that xrandr(1) can list connectors but it uses a
  different naming.

r274050:
  vt(4): Document kern.vt.fb.default_mode and kern.vt.fb.modes.*

  Those tunables are used to set a specific mode in vt(4) instead of using
  the default mode.

  Differential Revision:	https://reviews.freebsd.org/D1098
  Reviewed by:	ak@, emaste@, kwm@

r274051:
  vt(4): Improve the description of kern.vt.fb.modes.$connector

  Differential Revision:	https://reviews.freebsd.org/D1098
  Submitted by:	emaste@

r274053:
  vt(4): Start new sentences on their own lines

  Submitted by:	brueffer@

MFC of:		r274031, r274049, r274050, r274051, r274053

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