drm/msm/mdp5: Fix mdp5_cfg_init error return

If mdp5_cfg_init fails because of an unknown major version, a null pointer
dereference occurs.  This is because the caller of init expects error
pointers, but init returns NULL on error.  Fix this by returning the
expected values on error.

Fixes: 2e362e17 (drm/msm/mdp5: introduce mdp5_cfg module)
Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>