drm/mgag200: Embed instance of struct drm_device in struct mga_device

Following current best practice, the instance of struct drm_device is now
embedded in struct mga_device. The respective field has been renamed from
'dev' to 'base' to reflect the relationship. Conversion from DRM device is
done via upcast. Using dev_private is no longer possible.

The patch also open-codes drm_dev_alloc() and DRM device initialization
is now performed by a call to drm_device_init().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-14-tzimmermann@suse.de