drm/i915: Don't enable pipe/plane/VCO early (wait for DPMS on).

The existing code handling the DPMS ON event is much more careful to
ensure that these registers are enabled according to strict sequencing
requirements. Enabling these early in mode_set simply defeats that.

Signed-off-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Eric Anholt <eric@anholt.net>