drm/rockchip: dw-mipi-dsi: allow commands in panel_disable

Panel drivers may want to sent commands during the disable function, for
example MIPI_DCS_SET_DISPLAY_OFF before the video signal ends.  In order
to send commands we need to write to registers, so pclk must be enabled.

While changing this, remove the unnecessary code after the panel
unprepare call which seems to be a workaround for a specific panel and
thus belongs in the panel driver.

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/20170224125506.21533-13-john@metanate.com