drm/mst: use DP_GET_SINK_COUNT() for sink count in ESI

Take bit 7 into account when reading sink count from
DP_DEVICE_SERVICE_IRQ_VECTOR_ESI0.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220104184857.784563-2-jani.nikula@intel.com