Add a generic helper function drm_dp_lane_count_is_valid() to check if a DisplayPort lane count is valid. According to the DP specification, only 1, 2, or 4 lanes are supported. This helper avoids duplicating DP lane count validation logic across individual DisplayPort drivers. Suggested-by: Luca Ceresoli <luca.ceresoli@bootlin.com> Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com> Signed-off-by: Damon Ding <damon.ding@rock-chips.com> Link: https://patch.msgid.link/20260828065153.590802-3-damon.ding@rock-chips.com Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>