nouveau: bring back legacy mmap handler

nouveau userspace back at 1.0.1 used to call the X server
DRIOpenDRMMaster interface even for DRI2 (doh!), this attempts
to map the sarea and fails if it can't.

Since 884c6dab from Daniel,
this fails, but only ancient drivers would see it.

Revert the nouveau bits of that fix.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: <stable@vger.kernel.org>  # 3.18
Signed-off-by: Dave Airlie <airlied@redhat.com>