drm/msm/mdp4: add mdp axi clk

Downstream kernel holds this clk via a fake-parent relationship.
Upstream clock framework requires that we hold it explicitly.

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

diff --cc drivers/gpu/drm/msm/mdp/mdp4/mdp4_kms.c
Simple merge

diff --cc drivers/gpu/drm/msm/mdp/mdp4/mdp4_kms.h
Simple merge