drm/i915: Make sure DPLL is enabled when kicking the power sequencer on VLV/CHV

The power seqeuencer kick procedure requires the DPLL to be running
in order to complete successfully. In case the DPLL isn't currently
running when we need to kick the power seqeuncer enable it
temporarily. This can happen eg. during ->detect() when the pipe is
not already active.

To avoid needlessly duplicating the DPLL programming re-use the already
existing functions by passing a temporary pipe config to them instead
of having them consult the current pipe config at crtc->config.

v2: Introduce vlv_force_pll_{on,off}() (Daniel)
v3: Rebase due to drm_crtc vs. intel_crtc changes
    Fix a typo in commit msg (checkpatch)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com> (v1)
[danvet: Appease checkpatch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>