drm/gma500: Store framebuffer in struct drm_fb_helper

The gma500 driver stores the console framebuffer in struct psb_fbdev.
Moving it into struct drm_fb_helper will allow for removal of struct
psb_fbdev.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191122100545.16812-6-tzimmermann@suse.de