drm/dp/mst: Reduce nested ifs

Replace the nested ifs with a single if and a logical AND.

Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190925141442.23236-1-ville.syrjala@linux.intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>