drm: fix usage of vm_phys_fictitious_to_vm_page

vm_phys_fictitious_to_vm_page should not be called directly, even when
operating on a range that has been registered using
vm_phys_fictitious_reg_range. PHYS_TO_VM_PAGE should be used instead
because on arches that use VM_PHYSSEG_DENSE the page might come
directly from vm_page_array.

Reported by: nwhitehorn
Tested by: nwhitehorn, David Mackay <davidm.jx8p@gmail.com>
Sponsored by: Citrix Systems R&D
Set fb_pbase properly on PowerPC in the case where we have to guess at
the right register bank for the framebuffer. Disable the assigned-addresses
path on SPARC since it is just a hack for IBM PPC systems and was neither
relevant for nor worked on SPARC anyway.