msm: mdss: return a more meaningful value for dsi dcs command tx/rx

Return positive value (number of bytes sent or received) and 0
for failed cases for DSI command Tx and Rx functions for
consistency reason. Also update the return values of the APIs
that are dependent on the tx/rx functions.

CRs-Fixed: 704825
Change-Id: I6e8862c6987ee581f3baa7dce40da4e701cfaa2e
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>