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

drivers/gpu/drm/i915/i915_gem_context.h

@@ -46,6 +46,8 @@ struct intel_ring;
 
 #define DEFAULT_CONTEXT_HANDLE 0
 
+struct intel_context;
+
 struct intel_context_ops {
 	void (*unpin)(struct intel_context *ce);
 	void (*destroy)(struct intel_context *ce);