drm/i915: track dp target_clock in pipe_config

We need it in the fdi m_n computation, which nicely kills almost
all ugly special cases in there.

It looks like we also need this to handle 12bpc hdmi correctly.

Eventually it might be better to switch things around and put the
target clock into adjusted_mode->clock and create a new pipe_config
parameter for the port link clock.

v2: Add a massive comment in the code to explain this mess.

v3: s/dp_target_clock/pixel_target_clock in anticipation of the hdmi
use-case.

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