drm/i915: ignore link rate in TPS3 selection

TPS3 is mandatory for downstream devices that support HBR2, and Intel
platforms that support HBR2 also support TPS3. Whenever TPS3 is
supported by both the source and sink, it should be used. In other
words, whenever the source and sink are capable of 5.4 Gbps link, we
should anyway go for TPS3, regardless of the link rate being selected.

Log an error if the sink has advertized HBR2 capability without TPS3
capability.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>