drm/msm/mdp5: remove the cfg pointer from SMP struct

We want to make sure we control all the information being passed
down to SMP block. Having access to the cfg pointer here may create
bad things in the future.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>