drm/mgag200: Implement new init logic

Rework mgag200_regs_init() and mgag200_mm_init() into device preinit
and init functions. The preinit function, mgag200_device_preinit(),
requests and maps a device's I/O and video memory. The init function,
mgag200_device_init() initializes the state of struct mga_device.
Splitting the initialization between the two functions is necessary
to perform per-model operations between the two calls, such as reading
the unique revision ID on G200SEs.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601112522.5774-6-tzimmermann@suse.de