drm/i915/bxt: Restrict PORT_CLK_SEL programming below gen9

PORT_CLK_SEL programming is needed only on HSW/BDW.

v2:
- don't program PORT_CLK_SEL from mst encoders either (imre)
v3:
- fix the check for GEN9+ in intel_mst_pre_enable_dp() (damien)

Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sagar Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>