nouveau: support for custom VRAM domains

nouveau: support for custom VRAM domains

Some GPUs (e.g. GK20A, GM20B) do not embed VRAM of their own and use
the system memory as a backend instead. For such systems, allocating
objects in VRAM results in errors since the kernel will not allow
VRAM objects allocations.

This patch adds a vram_domain member to struct nouveau_screen that can
optionally be initialized to an alternative domain to use for VRAM
allocations. If left untouched, NOUVEAU_BO_VRAM will be used for
systems that embed VRAM, and NOUVEAU_BO_GART will be used for VRAM-less
systems.

Code that uses GPU objects is then expected to use the NV_VRAM_DOMAIN()
macro in place of NOUVEAU_BO_VRAM to ensure correct behavior on
VRAM-less chips.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Martin Peres <martin.peres@free.fr>