drm/fb-helper: don't sleep for screen unblank when an oops is in progress

Otherwise the system will burn even brighter and worse, leave the user
wondering what's going on exactly.

Since we already have a panic handler which will (try) to restore the
entire fbdev console mode, we can just bail out.  Inspired by a patch from
Konstantin Khlebnikov.  The callchain leading to this, cut&pasted from
Konstantin's original patch:

callstack:
panic()
bust_spinlocks(1)
unblank_screen()
vc->vc_sw->con_blank()
fbcon_blank()
fb_blank()
info->fbops->fb_blank()
drm_fb_helper_blank()
drm_fb_helper_dpms()
drm_modeset_lock_all()
mutex_lock(&dev->mode_config.mutex)

Note that the entire locking in the fb helper around panic/sysrq and kdbg
is ...  non-existant.  So we have a decent change of blowing up
everything.  But since reworking this ties in with funny concepts like the
fbdev notifier chain or the impressive things which happen around
console_lock while oopsing, I'll leave that as an exercise for braver
souls than me.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>