GEM does already a good job in tracking access to gem buffers via handles
and drm_vma access management. However, TTM drivers currently do not
verify this during mmap().

TTM provides the verify_access() callback to test this. So fix all drivers
to actually call into gem+vma to verify access instead of always returning
0.

All drivers assume that user-space can only get access to TTM buffers via
GEM handles. So whenever the verify_access() callback is called from
ttm_bo_mmap(), the buffer must have a valid embedded gem object. This is
true for all TTM+GEM drivers. But that's why this patch doesn't touch pure
TTM drivers (ie, vmwgfx).

v2: Switch to drm_vma_node_verify_access() to correctly return -EACCES if
    access was denied.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>