[IA64] agp/hp-agp: Allow binding user memory to the AGP GART

dmesg reports:
[   29.365973] [TTM] AGP Bind memory failed.
[   29.366015] radeon 0000:80:00.0: object_init failed for (4096, 0x00000002)
[   29.366052] radeon 0000:80:00.0: (-22) create WB bo failed
[   29.366087] radeon 0000:80:00.0: Disabling GPU acceleration
[   29.366124] [drm] radeon: cp finalized
[   29.366168] [drm] radeon: cp finalized
[   29.366210] [TTM] Finalizing pool allocator.
[   29.366924] [TTM] Zone  kernel: Used memory at exit: 0 kiB.
[   29.366961] [TTM] Zone   dma32: Used memory at exit: 0 kiB.
[   29.366996] [drm] radeon: ttm finalized
[   29.367030] [drm] Forcing AGP to PCI mode

This patch allows binding user memory to the AGP GART on zx1-based
systems. dmesg thus no more complains about AGP bind memory failure,
disabled GPU acceleration or AGP mode forced to PCI.

Original work from Francisco Jerez in agp/amd-k7
(https://lkml.org/lkml/2010/10/15/469

).

Tested-by: Émeric Maschino <emeric.maschino@gmail.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>