drm/msm/dsi-staging: fix bad pointer dereference in dsi_ctrl_get

If there is an attempt to obtain a 2nd reference count for a particular
dsi controller, the device is reported busy and the pointer value is
invalidated.  This is a problem since the structure that this pointer
refers to is holding a mutex lock at this point and when attempting
to unlock it the invalidated pointer value gets dereferenced.  This
should only occur if there is an invalid device tree configuration with
more than one active displays configured.

Change-Id: If4b55f8f927bb26990303bec1b830739de5e04ae
Signed-off-by: Steve Cohen <cohens@codeaurora.org>