drm/i915: Avoid ce->gem_context->i915

My plan for the future is to have kernel contexts not to have a GEM
context backpointer (as they will not belong to any GEM context). In a
few places, we use ce->gem_context to simply obtain the i915 backpointer,
for which we can use ce->engine->i915 instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730163441.16477-1-chris@chris-wilson.co.uk