drm/gma500: Remove struct psb_fbdev

Gma500's struct psb_fbdev is an, otherwise empty, wrapper around
struct drm_fb_helper. Remove it.

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/20191122100545.16812-7-tzimmermann@suse.de