drm: add initial_config function to fb helper

Rather than building a config which may or may not work, let the driver
build an initial fb config.  This allows the driver to use the BIOS boot
configuration for example, displaying kernel messages and the initial fb
console on the same outputs the BIOS lit up at boot time.  If that
fails, the driver can still fall back the same way as the core.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>