pvpanic: Set high notifier priority

We've observed the missing pvpanic call at panic, and it turned out
that this was blocked by the broken notifier of drm_fb_helper, where
scheduling may be called during switching to the fb console.
It's fairly difficult to fix the drm_fb problem and a quick fix isn't
foreseen, a simpler solution for the missing pvpanic call would be
just to call this earlier.

In order to assure that, this patch sets a higher priority to pvpanic
notifier_block.  Once when the issue of drm_fb is resolved, we can
remove this priority again.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>