PCI: Use is_visible() with boot_vga attribute for pci_dev

Should make pci_create_sysfs_dev_files() simpler.  Also fix possible
memleak in remove path.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>