drm/selftest: fix spelling mistake "dimention" -> "dimension"

There is a spelling mistake in a pr_err message, fix this.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>

drivers/gpu/drm/selftests/test-drm_damage_helper.c

@@ -53,7 +53,7 @@ static bool check_damage_clip(struct drm_plane_state *state, struct drm_rect *r,
 	int src_y2 = (state->src.y2 >> 16) + !!(state->src.y2 & 0xFFFF);
 
 	if (x1 >= x2 || y1 >= y2) {
-		pr_err("Cannot have damage clip with no dimention.\n");
+		pr_err("Cannot have damage clip with no dimension.\n");
 		return false;
 	}