drm/mgag200: Merge VRAM setup into MM initialization

The VRAM setup in mgag200_drv.c is part of memory management and
should be done in the same place. Merge the code into the memory
management's init function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-7-tzimmermann@suse.de