drm/i915: Clear shared dpll based on old state, v2.

drm/i915: Clear shared dpll based on old state, v2.

Atomic resume was preserving the dpll state because it was required
for clearing pll state correctly. If we look at the old_crtc_state
for pll to clear this is not needed and the hack can be removed.

Changes since v1:
- Rename dpll variable to old_dpll. (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455022343-15222-1-git-send-email-maarten.lankhorst@linux.intel.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>