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>

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -484,6 +484,7 @@ intel_ring_get_request(struct intel_engine_cs *ring)
  * will always have sufficient room to do its stuff. The request creation
  * code calls this automatically.
  */
+int intel_ring_reserve_space(struct drm_i915_gem_request *request);
 void intel_ring_reserved_space_reserve(struct intel_ringbuffer *ringbuf, int size);
 /* Cancel the reservation, e.g. because the request is being discarded. */
 void intel_ring_reserved_space_cancel(struct intel_ringbuffer *ringbuf);