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 <hofrat@osadl.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>