drm/rockchip: dw-mipi-dsi: remove mode_set hook

This is not needed since we can access the mode via the CRTC from the
enable hook.  Also remove the "mode" field that is no longer used.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170224125506.21533-4-john@metanate.com