drm/msm/dpu: Avoid a null de-ref while recovering from kms init fail

In the failure path for dpu_kms_init() it is possible to get to the MMU
destroy function with uninitialized MMU structs. Check for NULL and skip
if needed.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>