drm/msm/mdp5: Start using mdp5_hw_mixer

Use the mdp5_hw_mixer struct in the mdp5_crtc and mdp5_ctl instead of
using the LM index.

Like before, the Layer Mixers are assigned statically to the CRTCs.
The hwmixer(s) will later be dynamically assigned to CRTCs.

For now, ignore the hwmixers that can only do WB.

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