drm/i915: remove leaky eDP functions

Jesse Barnes noticed in his review of my DP cleanup series that
intel_edp_target_clock is now unused. Checking related code I've
noticed that also intel_edp_link_config is long unused.

Kill them both.

Wrt leaky eDP functions used in the common crtc code, the only thing
still left is intel_encoder_is_pch_edp. That one is just due to the
massive confusion between eDP vs. DP and port A vs. port D. Crtc code
should at most concern itself with the later, never with the former.

But that's material for another patch series.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>