drm/i915: Make i9xx_crtc_clock_get() work for PCH DPLLs

Add the 120MHz refernce clock case for PCH DPLLs.

Also determine the reference clock frequency more accurately by
checking for the PLLB_REF_INPUT_SPREADSPECTRUMIN refclk input
mode. The gen2 code already checked it, but it stil assumed a
fixed 66MHz refclk. Instead we need to consult the VBT for the
real value.

v2: Fix refclk for SSC panel case

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>