drm/i915: MCH_SSKPD is a 64 bit register on Haswell

And the SNB_READ_WM0_LATENCY macro is not valid anymore because we
have the "New WM0" at 63:56, so the "Old WM0" could maybe be zero if
the new one is not zero.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>