drm/i915: Add *_ring_begin() to request allocation

Now that the *_ring_begin() functions no longer call the request allocation
code, it is finally safe for the request allocation code to call *_ring_begin().
This is important to guarantee that the space reserved for the subsequent
i915_add_request() call does actually get reserved.

v2: Renamed functions according to review feedback (Tomas Elf).

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_lrc.h b/drivers/gpu/drm/i915/intel_lrc.h
index 044c0e5..f59940a 100644
--- a/drivers/gpu/drm/i915/intel_lrc.h
+++ b/drivers/gpu/drm/i915/intel_lrc.h
@@ -37,6 +37,7 @@
 
 /* Logical Rings */
 int intel_logical_ring_alloc_request_extras(struct drm_i915_gem_request *request);
+int intel_logical_ring_reserve_space(struct drm_i915_gem_request *request);
 void intel_logical_ring_stop(struct intel_engine_cs *ring);
 void intel_logical_ring_cleanup(struct intel_engine_cs *ring);
 int intel_logical_rings_init(struct drm_device *dev);