drm/gma500: Remove field 'fbdev' from struct psb_framebuffer

The field 'fbdev' in struct psb_framebuffer serves no purpose. 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/20191122100545.16812-3-tzimmermann@suse.de