drm/msm/mdp5: Add structs for hw Layer Mixers

Create a struct to represent MDP5 Layer Mixer instances. This will
eventually allow us to detach CRTCs from the Layer Mixers, and
generally clean things up a bit.

This is very similar to how hwpipes were previously abstracted away
from drm planes.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/Makefile

@@ -40,6 +40,7 @@ msm-y := \
 	mdp/mdp5/mdp5_mdss.o \
 	mdp/mdp5/mdp5_kms.o \
 	mdp/mdp5/mdp5_pipe.o \
+	mdp/mdp5/mdp5_mixer.o \
 	mdp/mdp5/mdp5_plane.o \
 	mdp/mdp5/mdp5_smp.o \
 	msm_atomic.o \