mdss: display: add cmdlist to tx/rx dcs command

Since dsi link is a shared resource between pixel stream and
dcs command, it needs a mechanism to protect dsi link to be accessed
exclusively. cmdlist is introduced for this purpose in this patch.

Change-Id: Ibfcab875c471c16e509cbf39c64c02f9423a6766
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>