parisc: agp/parisc-agp: allow binding of user memory to the AGP GART

Allow binding of user memory to the AGP GART on systems with HP
Quicksilver AGP bus. This resolves 'bind memory failed' error seen in
dmesg:

 [29.365973] [TTM] AGP Bind memory failed.
 …
 [29.367030] [drm] Forcing AGP to PCI mode

The system doesn't more fail to bind the memory, and hence not falling
back to the PCI mode (if other failures aren't detected).

This is just a simple write down from the following patches:
agp/amd-k7: Allow binding user memory to the AGP GART
agp/hp-agp: Allow binding user memory to the AGP GART

Signed-off-by: Alex Ivanov <gnidorah@p0n4ik.tk>
Cc: <stable@vger.kernel.org> # 3.10
Signed-off-by: Helge Deller <deller@gmx.de>