drm/msm: mdp5: Correctly return ERR_PTR for mdp5_cfg_init

The caller for mdp5_cfg_init expects a valid pointer or
a ERR_PTR encoded error.  Returning NULL leads to a kernel
oops.

Change-Id: Ic0dedbad44e37bdbc458fad6713d42ab249428d8
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>