drm/mipi-dsi: Avoid potential NULL pointer dereference

The mipi_dsi_packet_create() function dereferences the msg pointer
before checking that it's valid. Move the dereference down to where it
is required to avoid potentially dereferencing a NULL pointer.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>