nouveau: add PRIME support

This adds prime->fd and fd->prime support to nouveau,
it passes the SG object to TTM, and then populates the
GART entries using it.

v2: add stubbed kmap + use new function to fill out pages array
for faulting + add reimport test.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>