drm/i915: Mark up inline getters as taking a const i915_request

Since these inline routines only return the desired pointer from the
i915_request(after checking the preconditions for acquiring said
pointer), they can be const.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616183139.4061-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.h

@@ -561,7 +561,7 @@ static inline void i915_request_clear_hold(struct i915_request *rq)
 }
 
 static inline struct intel_timeline *
-i915_request_timeline(struct i915_request *rq)
+i915_request_timeline(const struct i915_request *rq)
 {
 	/* Valid only while the request is being constructed (or retired). */
 	return rcu_dereference_protected(rq->timeline,
@@ -569,14 +569,14 @@ i915_request_timeline(struct i915_request *rq)
 }
 
 static inline struct i915_gem_context *
-i915_request_gem_context(struct i915_request *rq)
+i915_request_gem_context(const struct i915_request *rq)
 {
 	/* Valid only while the request is being constructed (or retired). */
 	return rcu_dereference_protected(rq->context->gem_context, true);
 }
 
 static inline struct intel_timeline *
-i915_request_active_timeline(struct i915_request *rq)
+i915_request_active_timeline(const struct i915_request *rq)
 {
 	/*
 	 * When in use during submission, we are protected by a guarantee that