video/xen-fbfront: Further s/MFN/GFN clean-up
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 <2fe41d4e32b0bb991f63b975f2bed08c1e666665@citrix.com>
Reviewed-by: David Vrabel <b9d45bd1f671e508cd8daa63dfc3cabb597562df@citrix.com>
Signed-off-by: David Vrabel <b9d45bd1f671e508cd8daa63dfc3cabb597562df@citrix.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs