drm/fbdev-helper: Explain how to debug console_lock fun

drm/fbdev-helper: Explain how to debug console_lock fun

Every new KMS driver writer seems to run into this and wonder how
exactly drm_fb_helper_initial_config can die doing nothing at all.
Set up some big warnings signs around this newbie trap to avoid future
frustration and wasting everyone's time.

v2: Edits from Laurent.

Cc: Carlos Palminha <CARLOS.PALMINHA@synopsys.com>
Cc: Xinliang Liu <xinliang.liu@linaro.org>
Cc: laurent.pinchart@ideasonboard.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453449225-10954-1-git-send-email-daniel.vetter@ffwll.ch
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>