drm/rockchip: vop2: Add error check to devm_regmap_init_mmio

devm_regmap_init_mmio() may return an invalid pointer in case of an error.
This patch adds the corresponding IS_ERR check to vop2->map.

Signed-off-by: Alfredo Cruz <alfredo.carlon@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230322224411.15612-1-alfredo.carlon@gmail.com