We claim the ornamental design for a door panel, as shown and described.
FIG. 1 is a perspective view of a door panel of the claimed design.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is a left-side view thereof.
FIG. 5 is a right-side view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The dot-dashed broken lines along the edges of the design depict the bounds of the claim. The dashed broken lines depict environmental subject matter. All broken lines form no part of the claim.