drm: Remove utterly bogus preempt_disable() sections

commit 27641c3f (drm/vblank: Add support for precise vblank
timestamping) adds preempt_disable()/enable() around a spin locked
section with the comments:

 * Disable preemption, so vblank_time_lock is held as short as
 * possible, even under a kernel with PREEMPT_RT patches.

/* Disable preemption while holding vblank_time_lock. Do
 * it explicitely to guard against PREEMPT_RT kernel.

Just that this has never been tested on a RT kernel which would have
granted that nonsense with a might_sleep() warning because
dev->vblank_time_lock is converted to a "sleeping" spinlock on RT.

So this is activly wrong on RT and superflous on mainline. Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>