drm/vmwgfx: Drop preempt_disable() in vmw_fifo_ping_host()

drm/vmwgfx: Drop preempt_disable() in vmw_fifo_ping_host()

vmw_fifo_ping_host() disables preemption around a test and a register
write via vmw_write(). The write function acquires a spinlock_t typed
lock which is not allowed in a preempt_disable()ed section on
PREEMPT_RT. This has been reported in the bugzilla.

It has been explained by Thomas Hellstrom that this preempt_disable()ed
section is not required for correctness.

Remove the preempt_disable() section.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=206591
Link: https://lkml.kernel.org/r/0b5e1c65d89951de993deab06d1d197b40fd67aa.camel@vmware.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>