drm/hisilicon: hibmc: Don't overwrite fb helper surface depth

Currently the driver overwrites the surface depth provided by the fb
helper to give an invalid bpp/surface depth combination.

This has been exposed by commit 70109354 ("drm: Reject unknown legacy
bpp and depth for drm_mode_addfb ioctl"), which now causes the driver to
fail to probe.

Fix by not overwriting the surface depth.

Fixes: d1667b86

 ("drm/hisilicon/hibmc: Add support for frame buffer")
Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Xinliang Liu <z.liuxinliang@hisilicon.com>
Signed-off-by: Xinliang Liu <z.liuxinliang@hisilicon.com>