drm/<ttm-based-drivers>: Don't call drm_mmap

Really, the legacy buffer api should be dead, especially for all these
newfangled drivers. I suspect this is copypasta from the transitioning
days, which probably originated in radeon.

Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Rashika <rashika.kheria@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Alexandre Courbot <acourbot@nvidia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>