drm/i915: Avoid TP3 on CHV

This patch removes TP3 support on CHV since there is no support
for HBR2 on this platform.

v2: rename the function to indicate it checks source rates (Jani)
v3: update comment to indicate TP3 dependency on HBR2 supported
    hardware (Jani)

Cc: stable@vger.kernel.org # v4.1+
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
[Jani: fixed a couple of checkpatch warnings.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>