drm/rockchip: dw-mipi-dsi: fix command header writes

In a couple of places here we use "val" for the value that is about to
be written to a register but then reuse the same variable for the value
of a status register before we get around to writing it.  Rename the
value to be written to so that we write the value we intend to and not
what we have just read from the status register.

Signed-off-by: John Keeping <john@metanate.com>
Tested-by: Chris Zhong <zyw@rock-chips.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/20170224125506.21533-5-john@metanate.com