drm/gma500: Pass fb_info to psb_fbdev_vm_fault()

Instead of the DRM framebuffer, pass the FB info strcuture to the
fbdev page-fault handler psb_fbdev_vm_fault(). The framebuffer is a
high-level data structure and does not belong into fault handling.
The fb_info has all necessary information. Also set fix.smem_start
to the correct value (the beginning of the framebuffer in physical
address space) and streamline the page-fault handler.

v2:
	* remove unused struct drm_psb_private.fb_base (Patrik)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230313151610.14367-8-tzimmermann@suse.de