drm/i915: Avoid overflowing the DP link rate arrays

Complain loudly if we ever attempt to overflow the the supported_rates[]
array. This should never happen since the sink_rates[] array will always
be smaller or of equal size. But should someone change that we want to
catch it without scribblign over the stack.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>