Same fix as rB0a3b4d4c64f1, but this time for greasepencil.
To repeat: dopesheet in greasepencil mode was ignoring the temporariy
visibility flag of collections. As a result, even though the dopesheet
was supposed to show animation data of visible greasepencils only was
still showing such data of greasepencils that were hidden by hiding
their collection.
b20997cb34