video/xen-fbfront: Further s/MFN/GFN clean-up

The PV driver xen-fbfront is only dealing with GFN and not MFN. Rename
all the occurence of MFN to GFN.

Also take the opportunity to replace to usage of pfn_to_gfn by
xen_page_to_gfn.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>