drm/i915: move dp_m_n computation to dp_encoder->compute_config

We need a flag to designate dp encoders and the dp link m_n parameters
in the pipe config for that. And now that the pipe bpp computations
have been moved up and stored in the pipe config, too, we can do this
without losing our sanity.

v2: Rebased on top of Takashi Iwai's fix to (again) fix the target
clock handling for eDP. Luckily the new code is sane enough and just
does the right thing!

v3: Move ->has_dp_encoder to this patch (Jesse).

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