drm/i915: Generalize intel_ring_get_tail to take a ringbuf

Again, it's low-level enough to simply take a ringbuf and nothing
else.

Trivial change.
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NOscar Mateo <oscar.mateo@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2330,7 +2330,7 @@ int __i915_add_request(struct intel_engine_cs *ring,
 	u32 request_ring_position, request_start;
 	int ret;
 
-	request_start = intel_ring_get_tail(ring);
+	request_start = intel_ring_get_tail(ring->buffer);
 	/*
 	 * Emit any outstanding flushes - execbuf can fail to emit the flush
 	 * after having emitted the batchbuffer command. Hence we need to fix
@@ -2351,7 +2351,7 @@ int __i915_add_request(struct intel_engine_cs *ring,
 	 * GPU processing the request, we never over-estimate the
 	 * position of the head.
 	 */
-	request_ring_position = intel_ring_get_tail(ring);
+	request_ring_position = intel_ring_get_tail(ring->buffer);
 
 	ret = ring->add_request(ring);
 	if (ret)

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -386,9 +386,9 @@ int intel_init_vebox_ring_buffer(struct drm_device *dev);
 u64 intel_ring_get_active_head(struct intel_engine_cs *ring);
 void intel_ring_setup_status_page(struct intel_engine_cs *ring);
 
-static inline u32 intel_ring_get_tail(struct intel_engine_cs *ring)
+static inline u32 intel_ring_get_tail(struct intel_ringbuffer *ringbuf)
 {
-	return ring->buffer->tail;
+	return ringbuf->tail;
 }
 
 static inline u32 intel_ring_get_seqno(struct intel_engine_cs *ring)