drm/amdgpu: fix signed overrun in amdgpu_ctx_get_fence

Otherwise the first 16 fences of a context will always signal immediately.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c

@@ -226,7 +226,7 @@ struct fence *amdgpu_ctx_get_fence(struct amdgpu_ctx *ctx,
 		return ERR_PTR(-EINVAL);
 	}
 
-	if (seq < cring->sequence - AMDGPU_CTX_MAX_CS_PENDING) {
+	if (seq + AMDGPU_CTX_MAX_CS_PENDING < cring->sequence) {
 		spin_unlock(&ctx->ring_lock);
 		return NULL;
 	}