drm/i915: move i9xx dpll enabling into crtc enable function

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>