drm/rockchip/dsi: remove mode_valid function

The MIPI DSI do not need check the validity of resolution, the max
resolution should depend VOP. Hence, remove rk3288_mipi_dsi_mode_valid
here.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1487577744-2855-5-git-send-email-zyw@rock-chips.com