drm/i915: Fully separate source vs. sink rates

Remove the sink vs. source limit mess from intel_dp_max_link_bw() and
just move the source restriction checks to intel_dp_source_rates().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
[danvet: Resolve conflict with WaDisableHBR2:skl patch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>