drm/i915: clean up vlv ->pre_pll_enable and pll enable sequence

No need to call the ->pre_pll_enable hook twice if we don't enable the
dpll too early. This should make Jani a bit less grumpy.

v2: Rebase on top of the newly-colored BUG_ONs.

v3: Reinstate the lost write of the DPLL_MD register, spotted by Imre.

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