drm/msm/a5xx: fix the emptyness check in the preempt code

Quoting Yassine: ring->memptrs->rptr is never updated and stays 0, so
the comparison always evaluates to false and get_next_ring always
returns ring 0 thinking it isn't empty.

Fix this by calling get_rptr() instead of reading rptr directly.
Reported-by: Yassine Oudjana <y.oudjana@protonmail.com>
Fixes: b1fc2839 ("drm/msm: Implement preemption for A5XX targets")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/522642/
Link: https://lore.kernel.org/r/20230214020956.164473-4-dmitry.baryshkov@linaro.orgSigned-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/adreno/a5xx_preempt.c

@@ -63,7 +63,7 @@ static struct msm_ringbuffer *get_next_ring(struct msm_gpu *gpu)
 		struct msm_ringbuffer *ring = gpu->rb[i];
 
 		spin_lock_irqsave(&ring->preempt_lock, flags);
-		empty = (get_wptr(ring) == ring->memptrs->rptr);
+		empty = (get_wptr(ring) == gpu->funcs->get_rptr(gpu, ring));
 		spin_unlock_irqrestore(&ring->preempt_lock, flags);
 
 		if (!empty)