drm/dsi: Add message to packet translator

[Dan Carpenter]

Hello Thierry Reding,

This is a semi-automatic email about new static checker warnings.

The patch a52879e8d7cb: "drm/dsi: Add message to packet translator" 
from Oct 16, 2014, leads to the following Smatch complaint:

drivers/gpu/drm/drm_mipi_dsi.c:328 mipi_dsi_create_packet()
         warn: variable dereferenced before check 'msg' (see line 326)

drivers/gpu/drm/drm_mipi_dsi.c
   325  {
   326          const u8 *tx = msg->tx_buf;
                               ^^^^^^^^^^^
Dereference.

   327  
   328          if (!packet || !msg)
                               ^^^^
Check.

   329                  return -EINVAL;
   330  

regards,
dan carpenter