drm/i915/mst: read link status only when requested by sink in ESI

The link service irq vector in DPCD 0x2005 contains the link status
changed bit to indicate the status should be checked. Only read and
check the link status when requested by the sink.

This also reduces the confusion around the buffer size for the combined
ESI and link status. Alas, we still need to take into account that all
link status helpers expect a buffer of DP_LINK_STATUS_SIZE (6) while the
link status in ESI only has 4 bytes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220112110319.1172110-5-jani.nikula@intel.com