drm/i915: Depend on !PREEMPT_RT.

There are a few sections in the driver which are not compatible with
PREEMPT_RT. They trigger warnings and can lead to deadlocks at runtime.

Disable the i915 driver on a PREEMPT_RT enabled kernel. This way
PREEMPT_RT itself can be enabled without needing to address the i915
issues first. The RT related patches are still in RT queue and will be
handled later.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/YgqmfKhwU5spS069@linutronix.de