drm/rockchip: vop: fix format bpp calculation

We can't rely on cpp for bpp calculation as the cpp of
some formats(DRM_FORMAT_YUV420_8BIT/10BIT, etc) is zero.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231018094210.2475771-1-andyshrk@163.com