drm/i915/bxt: Determine programmed frequency

Add placeholder function for calculating programmed pixel clock.
Note: Formula to back calculate link clock from dividers not
available currently.

v2:
- rebased on upstream s/crtc_config/crtc_state/ change (imre)

Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>