vgaarb: Stub vga_set_legacy_decoding()

vga_set_legacy_decoding() is defined in drivers/gpu/vga/vgaarb.c,
which is only compiled with CONFIG_VGA_ARB.  A caller would
therefore get an undefined symbol if the VGA arbiter is not
enabled.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Dave Airlie <airlied@redhat.com>