drm/rockchip: dw-mipi-dsi: fix escape clock rate

This clock rate is derived from the PHY PLL, so it should be calculated
dynamically.  This calculation is the same as that used by the vendor
kernel and ensures that the escape clock runs at <20MHz as required by
the MIPI specification.

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/20170224125506.21533-14-john@metanate.com