PCI/VGA: Factor out vga_select_framebuffer_device()

On x86 and ia64, if a VGA device BARs include a framebuffer reported by
platform firmware, we select the device as the default VGA device.  Factor
this code to a separate function.  No functional change intended.

Link: https://lore.kernel.org/r/20220224224753.297579-4-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Bruno Prémont <bonbons@linux-vserver.org>