drm/i915: Add cdclk extraction for g33, g965gm and g4x

Implement cdclk extraction for g33, 965gm and g4x platforms. The details
came from configdb. Sadly there isn't anything there for other gen3/gen4
chipsets.

So far I've tested this on one ELK where it gave me a HPLL VCO of 5333
MHz and cdclk of 444 MHz which seems perfectly sane for this machine.

v2: Rebased to the latest
v3: Rebased to the latest

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Acked-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>