drm/rockchip: Correct vop out_mode configure

Force vop output mode on encoder driver seem not a good idea,

EDP, HDMI, DisplayPort all have 10bit input on rk3399,
On non-10bit vop, vop 8bit output bit[0-7] connect to the
encoder high 8bit [2-9].

So force RGB10 to RGB888 on vop driver would be better.

And another problem, EDP check crtc id on atomic_check,
but encoder maybe NULL, so out_mode configure would fail,
it cause edp no display.

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1495885416-22216-1-git-send-email-mark.yao@rock-chips.com