drm/msm/mdp4: use msm_kms_init_vm() instead of duplicating it

Use the msm_kms_init_vm() function to allocate memory manager instead of
hand-coding a copy of it. Although MDP4 platforms don't have MDSS
device, it's still safe to use the function as all MDP4 devices have
IOMMU and the parent of the MDP4 is the root SoC device.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/672563/


Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>