drm/i915/bxt: BXT clock divider calculation

Calculate and cache clock parameters. Follow bspec algorithm for HDMI.
Use precalculated values for DisplayPort linkrates.

v2: (imre)
- rebase against upstream crtc_state change
- use the existing CHV based helper instead of handrolling the same

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>