drm/i915: kill intel_dp_link_clock()

Use drm_dp_bw_code_to_link_rate insead. It's the same thing, but
supports DP_LINK_BW_5_4 and is also used by the other drivers.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>