video: hyperv: hyperv_fb: refresh the VM screen by force on VM panic

Currently the VSC has no chance to notify the VSP of the dirty rectangle on VM
panic because the notification work is done in a workqueue, and in panic() the
kernel typically ends up in an infinite loop, and a typical kernel config has
CONFIG_PREEMPT_VOLUNTARY=y and CONFIG_PREEMPT is not set, so a context switch
can't happen in panic() and the workqueue won't have a chance to run. As a
result, the VM Connection window can't refresh until it's closed and we
re-connect to the VM.

We can register a handler on panic_notifier_list: the handler can notify
the VSC and switch the framebuffer driver to a "synchronous mode", meaning
the VSC flushes any future framebuffer change to the VSP immediately.

v2: removed the MS-TFS line in the commit message
v3: remove some 'unlikely' markings
v4: avoid global variables as Tomi Valkeinen suggested

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>