drm/i915: Leave DPLL ref clocks on

We enable the DPLL refclock already when bringing up the cmnlane power
well, so also leave it on when otherwise disabling the DPLL.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>