drm/arm/malidp: replace drm->dev_private with drm_to_malidp()

Using drm_device->dev_private is deprecated. Since we've switched to
devm_drm_dev_alloc(), struct drm_device is now embedded in struct
malidp_drm, hence we can use container_of() to get the struct drm_device
instance instead.

Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026155934.125294-3-dakr@redhat.com