drm/i915: Push the use-semaphore marker onto the intel_context

Instead of rummaging through the intel_context to peek at the GEM
context in the middle of request submission to decide whether to use
semaphores, store that information on the intel_context itself.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191220101230.256839-2-chris@chris-wilson.co.uk