drm/amdgpu: Check num_gfx_rings for gfx v9_0 rb setup.

stable inclusion
from stable-v5.10.143
commit 04102568671ee08d9de6a3e70db34cbd5eaa8228
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6D0U6

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=04102568671ee08d9de6a3e70db34cbd5eaa8228



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[ Upstream commit c3519383 ]

No need to set up rb when no gfx rings.

Signed-off-by: Candice Li <candice.li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>
Reviewed-by: Zheng Zengkai <zhengzengkai@huawei.com>