drm/i915: Refactor tests for validity of RING_TAIL

Whilst I like having the assertions clearly visible in the code, they
are quite repetitious! As we find new limits we want to incorporate into
the set of assertions, it make sense to refactor them to a common
routine.

v2: Add a guc holdout.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170327131412.20293-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -774,8 +774,7 @@ static void i9xx_submit_request(struct drm_i915_gem_request *request)
 
 	i915_gem_request_submit(request);
 
-	GEM_BUG_ON(!IS_ALIGNED(request->tail, 8));
-	GEM_BUG_ON(request->tail >= request->ring->size);
+	assert_ring_tail_valid(request->ring, request->tail);
 	I915_WRITE_TAIL(request->engine, request->tail);
 }
 
@@ -787,8 +786,7 @@ static void i9xx_emit_breadcrumb(struct drm_i915_gem_request *req, u32 *cs)
 	*cs++ = MI_USER_INTERRUPT;
 
 	req->tail = intel_ring_offset(req, cs);
-	GEM_BUG_ON(!IS_ALIGNED(req->tail, 8));
-	GEM_BUG_ON(req->tail >= req->ring->size);
+	assert_ring_tail_valid(req->ring, req->tail);
 }
 
 static const int i9xx_emit_breadcrumb_sz = 4;
@@ -827,8 +825,7 @@ static void gen8_render_emit_breadcrumb(struct drm_i915_gem_request *req,
 	*cs++ = MI_NOOP;
 
 	req->tail = intel_ring_offset(req, cs);
-	GEM_BUG_ON(!IS_ALIGNED(req->tail, 8));
-	GEM_BUG_ON(req->tail >= req->ring->size);
+	assert_ring_tail_valid(req->ring, req->tail);
 }
 
 static const int gen8_render_emit_breadcrumb_sz = 8;