Fix a typo that led to some MST short pulse event handling issue (the
short pulse event was handled for both encoder instances, each having
its own state).

Fixes: 1d8ca002 ("drm/i915: Add PORT_TCn aliases to enum port")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201104010000.4165574-1-imre.deak@intel.com