drm/i915: remove pre_pll_enable hook from DDI/gen9+ crtc enable

The hook was added to cater for DSI, but with the hooks rearranged on
the DSI encoder side, this is no longer needed. It was a bit silly
anyway to have two hooks called back-to-back.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448619706-21293-2-git-send-email-jani.nikula@intel.com