drm/i915/tv: Remove ->mode_set callback

drm/i915/tv: Remove ->mode_set callback

Currently for the i9xx crtc hooks there's nothing between the call to
encoder->mode_set and encoder->pre_enable which touches the hardware.

Therefore, since tv is only used on gen3/4, we can just move the hook.
Yay for easy cases!

The only other important thing to check is that the new
->pre_enable hook is idempotent wrt the sw state since now it can
be called multiple times (due to DPMS). After a the bit of refactoring
this is now easy to check: It only reads crtc->config and computes
derived state but otherwise leaves it as-is, so we're good.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>