drm/i915/mst: debug log 4 bytes of ESI right after reading

For whatever reason, the ESI link service irq vector was missing from
the debug output. Add the missing byte, clean up the debug message, and
do the logging right after reading the data.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220112110319.1172110-3-jani.nikula@intel.com