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

[ Upstream commit e4337877c5d578722c0716f131fb774522013cf5 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>