MFC r254882, r254883, r255571:

vga_pci: Add API to map the Video BIOS

Here are two new functions to map and unmap the Video BIOS:
    void * vga_pci_map_bios(device_t dev, size_t *size);
    void   vga_pci_unmap_bios(device_t dev, void *bios);

The BIOS is either taken from the shadow copy made by the System BIOS at
boot time if the given device was used for the default display (i386,
amd64 and ia64 only), or from the PCI expansion ROM.

Additionally, one can determine if a given device was the default
display at boot time using the following new function:
    int    vga_pci_is_boot_display(device_t dev);

Submitted by:	jhb@ (r255571)
MFC r259679:

vga_pci: Improve boot display detection

The previous code was checking the "VGA Enable" bit on the video card's
parent PCI-to-PCI bridge only. This didn't work for the case where the
video card is attached to the root PCI bus (ie. the card has no parent
PCI-to-PCI bridge).

Now, the new code:
    1. checks the "VGA Enable" bit on the parent bridge only if it's a
       PCI-to-PCI bridge;
    2. always checks the "I/O" and "Memory address space decoding" bits
       on the video card itself.

However, vendor-specific bits are not used.

This fixes the use of many integrated Radeon cards: without this patch,
we fail to detect them as the boot display and, when radeonkms looks for
the Video BIOS, it skips the shadow copy made by the System BIOS. It
then fails to fully initialize the card, because the shadow copy is the
only way to read the Video BIOS in these situations. A workaround was to
force the boot display selection using the "hw.pci.default_vgapci_unit"
tunable.

A previous version of this patch added a new function doing the checks.
Now, the vga_pci_is_boot_display() function is used to perform the
checks (only until the boot display is found) and return if the given
device is the boot display or not.

Furthermore, vga_pci_attach() logs "Boot video device" if the card being
attached it the Chosen One:
    vgapci0: <VGA-compatible display> [...]
    vgapci0: Boot video device

Reviewed by:	kib@, jhb@ (both a previous version)
Tested by:	lunatic_ (#freebsd-xorg, integrated Radeon card,
		xmj (#freebsd-xorg, i915+NVIDIA cards)