drm: avoid switching to text console if there is no panic timeout

Add a check for panic_timeout in the drm_fb_helper_panic() notifier: if
we're going to reboot immediately, the user will not be able to see the
messages anyway, and messing with the video mode may display artifacts,
and certainly get into several layers of complexity (including mutexes and
memory allocations) which we shall be much safer to avoid.

Signed-off-by: Hugh Dickins <hughd@google.com>
[ Edited commit message and modified to short-circuit panic_timeout < 0
  instead of testing panic_timeout >= 0.  -Mandeep ]
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>