usb: typec: altmodes: displayport: Supply missing displayport.h include file

usb: typec: altmodes: displayport: Supply missing displayport.h include file

If the header file containing a function's prototype isn't included by
the sourcefile containing the associated function, the build system
complains of missing prototypes.

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

 drivers/usb/typec/altmodes/displayport.c:511:5: warning: no previous prototype for ‘dp_altmode_probe’ [-Wmissing-prototypes]
 drivers/usb/typec/altmodes/displayport.c:551:6: warning: no previous prototype for ‘dp_altmode_remove’ [-Wmissing-prototypes]

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200703174148.2749969-22-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>