drm/i915: Check intel_contexts to avoid one extra pointer chase

As we store the intel_context on the request (rq->hw_context), we can
simply compare that against the local intel_context for the
i915->kernel_context rather than using the rq->gem_context.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180601094002.13329-1-chris@chris-wilson.co.uk