drm/ast: Replace driver load/unload functions with device create/destroy

The ast driver's load and unload functions are left-overs from when
struct drm_driver.load/unload was still in use. The PCI probe helper
allocated the DRM device and ran load to initialize it.

This patch replaces this code with device create and destroy. The
main difference is that the device's create function allocates the
DRM device and ast structures in the same place. This will be required
for switching ast to managed allocations.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-4-tzimmermann@suse.de