drm/gma500: Replace struct psb_framebuffer with struct drm_framebuffer

After removing all unnecessary fields, struct psb_framebuffer is just a
wrapper around struct drm_framebuffer. So we can replace the former with
the latter.

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/20191122100545.16812-4-tzimmermann@suse.de