drm/msm/mdp: Detach iommu in mdp4_destroy

attach_dev gets called in mdp4_kms_init, but there is no corresponding
detach_dev called in the error path or in the kms driver unload path.

Detach and destroy mmu in mdp4_destroy.

Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>