drm/mgag200: Allocate device structures in mgag200_driver_load()

Instances of struct drm_device and struct mga_device are now allocated
next to each other in mgag200_driver_load(). Yet another preparation
before embedding the DRM device instance in struct mga_device.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-13-tzimmermann@suse.de