vga: improve VGA logic

Improve VGA selection logic, push check for device availabilty to vl.c.
Create the devices at board level unconditionally.

Remove now unused pci_try_create*() functions.

Make PCI VGA devices optional.

Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
