drm/nouveau/pci: Handle 5-bit and 8-bit tag field

If the hardware supports extended tag field (8-bit ones), then enable it.

This is usually done by the VBIOS, but not on some MBPs (see fdo#86537).

In case extended tag field is not supported, 5-bit tag field is used which
limits the possible number of requests to 32. Apparently bits 7:0 of
0x08841c stores some number of outstanding requests, so cap it to 32 if
extended tag is unsupported.

Fixes: fdo#86537

v2: Restrict changes to chipsets >= 0x84
v3:
  * Add nvkm_pci_mask to pci.h
  * Mask bit 8 before setting it
v4:
  * Rename `add` argument of nvkm_pci_mask to `value`
  * Move code from nvkm_pci_init to g84_pci_init and remove PCIe and chipset
    checks
v5:
  * Rebase code on latest PCI structure
  * Restore PCIe check
  * Fix namings in nvkm_pci_mask
  * Rephrase part of the commit message

Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>