drm/v3d: HACK: gut runtime pm for now.

Something is still unstable -- on starting a new glxgears from an idle
X11, I get an MMU violation in high addresses.  The CTS also failed
quite quickly.  With this, CTS progresses for an hour before OOMing
(allocating some big buffers when my board only has 600MB available to
Linux)

Signed-off-by: Eric Anholt <eric@anholt.net>