drm/v3d: Hook up the runtime PM ops.

In translating the runtime PM code from vc4, I missed the ".pm"
assignment to actually connect them up.  Fixes missing MMU setup if
runtime PM resets V3D.

Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit ca197699af29baa8236c74c53d4904ca8957ee06)