Now that we have the proper pipe config to track this, we don't need
to write any registers any more.

Note that for platforms without DPLL_MD (pre-gen4) which store the
pixel mutliplier in the DPLL register I've decided to keep the
seemingly "redundant" write: The comment right below saying "do this
trice for luck" doesn't instill confidence ...

v2: Drop a few now unnecessary local variables and switch the enable
function to take a struct intel_crtc * to simply arguments.

v3: Rebase on top of the newly-colored BUG_ON.

v4: Amend commit message to alliviate Imre's comment about the
redudant DPLL write for the pixel mutliplier.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>