drm/msm/mdp5: Fix mdp5_init error path for failed mdp5_kms allocation

When allocation for mdp5_kms fails, calling mdp5_destroy() leads to undefined
behaviour, likely a nullptr exception or use-after-free troubles.

Signed-off-by: Roy Spliet <nouveau@spliet.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>