VLV wants encoder enabling before the pipe is up. With the previously
rearranged VLV DP and HDMI ->pre_enable and ->enable callbacks in place,
this no longer depends on the early ->enable hook call. Move the
->enable call at the end of the sequence, similar to the crtc enable on
other platforms. This will be needed e.g. for moving the eDP backlight
enabling to the right place in the sequence, currently done too early on
VLV.

There should be no functional changes.

v2: Rebase.

v3: Explain why this is needed in the commit message (Chris).
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>