drm/i915: Store preemption capability in engine->flags

drm/i915: Store preemption capability in engine->flags

Let's avoid having to delve down the pointer chain to see if the i915
device has support for preemption and store that on the engine, which
made the decision in the first place!

v2: Refactor common preemption policy between execlists/guc.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Tomasz Lis <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Cc: Michał Winiarski <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Daniele Ceraolo Spurio <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]