i915: switch back to fbcon on panic

Normally when X is running, panic messages will be invisible and the machine
will just appear to hard hang.  This patch adds support for switching back to
the fbcon framebuffer on panic (through the use of a panic notifier
registration) so we can see what happened.

Note that in order to be really useful, X will have to run its VT in something
other than KD_GRAPHICS mode.  Also, not all kernel errors result in panics,
some go through BUG() which may trigger another type of event, not resulting in
a switch.