The ornamental design for a door panel, as shown and described.
FIG. 1 is a front elevational view of the door panel showing our new design; and,
FIG. 2 is an upper perspective view thereof.
The uneven line adjacent to the armrest and immediately above the shaded area represents an unclaimed boundary of the design. The broken line showings of the control buttons on the armrest, the speaker, the speaker grill border, the door handle, the lock/unlock button, and the trim above the armrest are for illustrative purposes only and form no part of the claimed design.