drm/i915/hdmi: Remove ->mode_set callback

Similar to dp the only thing we do is call intel_write_eld and prepare
a bit of state for the enable hooks. The only difference is that we
write that to the hardware instead of keeping track of it somewhere in
software.

Still we can just move all this to the very first enable hook.

Reviewed-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>