drm/msm/edp: fix build warning - missing prototype

Fix build warning when building edp/edp_aux.o due to missing
prototype for edp_aux_transfer.

This function is only used in edp_aux.c so just make it static.

Reported-by: Nicholas Mc Guire <[e-mail]>
Signed-off-by: Rob Clark <[e-mail]>

drivers/gpu/drm/msm/edp/edp_aux.c

···
  * msm_edp_aux_ctrl() running concurrently in other threads, i.e.
  * start transaction only when AUX channel is fully enabled.
  */
-ssize_t edp_aux_transfer(struct drm_dp_aux *drm_aux, struct drm_dp_aux_msg *msg)
+static ssize_t edp_aux_transfer(struct drm_dp_aux *drm_aux,
+		struct drm_dp_aux_msg *msg)
 {
 	struct edp_aux *aux = to_edp_aux(drm_aux);
 	ssize_t ret;