The ornamental design for a driver's support cushion, as shown and described.
FIG. 1 is a perspective view of a driver's support cushion showing my new
design, the broken-line depiction of the automobile seat being for
illustrative purposes only and forming no part of the claimed design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a front elevational view thereof, the rear being a mirror image
of that shown;
FIG. 4 is a top plan view thereof; and
FIG. 5 is a side elevational view thereof, the opposite side being a mirror
image of that shown.