drm/i915: rearrange vlv hdmi enable and pre_enable callbacks

VLV wants encoder enabling before the pipe is up. This is currently
achieved through calling the ->enable callback early, right after the
->pre_enable callback, in valleyview_crtc_enable(). This loses both the
distinction between ->pre_enable and ->enable on VLV and the possibility
to use a hook at the end of the modeset sequence.

Rearrange the HDMI callbacks to make it possible to move ->enable call
later. Basically do everything in ->pre_enable on VLV, and make ->enable
a NOP.

There should be no functional changes.

v2: Rebase.

v3: Explain why this is needed in the commit message (Chris).

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>