drm/mgag200: Lookup VRAM PCI BAR start and length only once

The MM setup code on mgag200 reads PCI BAR 0's start and length
several times. Reusing these values makes the code more readable.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-6-tzimmermann@suse.de