drm/i915: remove unused power_well/get_cdclk_freq api

After switching to using the component interface this API isn't needed
any more.

v2-3: unchanged
v4:
- move the removal of i915_powerwell.h to this patch (Takashi)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>