usb: typec: altmodes: displayport: File headers are not good candidates for kerneldoc

usb: typec: altmodes: displayport: File headers are not good candidates for kerneldoc

Demote displayport's file header to a standard comment block.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/typec/altmodes/displayport.c:18: warning: Function parameter or member '_dp' not described in 'DP_HEADER'
 drivers/usb/typec/altmodes/displayport.c:18: warning: Function parameter or member 'cmd' not described in 'DP_HEADER'

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200703174148.2749969-21-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>