drm: rp1: VEC and DPI drivers: Fix bug #5901

Rework probe() to use devm_drm_dev_alloc(), embedding the DRM
device in the DPI or VEC device as now seems to be recommended.

Change order of resource allocation and driver initialization.
This prevents it trying to write to an unmapped register during
clean-up, which previously could crash.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>