drm/rockchip: dsi: Fix VOP selection on SoCs that support it

lcdsel_grf_reg is defined as u32, so "< 0" comaprison is always false,
which breaks VOP selection on eg. RK3399. Compare against 0.

Fixes: f3aaa612 ("drm/rockchip: dsi: add rk3568 support")
Signed-off-by: Ondrej Jirman <megi@xff.cz>
Tested-by: Chris Morgan <macromorgan@hotmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221023160747.607943-1-megi@xff.cz