drm/rockchip: dw-mipi-dsi: use specific poll helper

As the documentation for readx_poll_timeout says, we want to use the
specialized macro for readl rather than using the generic version
directly.

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/20170224125506.21533-19-john@metanate.com