drm/i915/cnl: Get DDI clock based on PLLs.

PLLs are the source clocks for the DDIs so in order
to determine the ddi clock we need to check the PLL
configuration.

v2: Mika pointed out that 24 was hardcoded while it
    should consider ref clock that can be either 24KHz
    or 19.2KHz on CNL.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1499374321-31152-1-git-send-email-rodrigo.vivi@intel.com