drm/arm/hdlcd: replace drm->dev_private with drm_to_hdlcd_priv()

Using drm_device->dev_private is deprecated. Since we've switched to
devm_drm_dev_alloc(), struct drm_device is now embedded in struct
hdlcd_drm_private, 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/20221026153431.72033-3-dakr@redhat.com