drm/i915: Forward declare struct intel_context

This is to avoid an error with structure declared in parameter list if the
include ordering changes.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180524150621.17332-2-tvrtko.ursulin@linux.intel.com