drm/msm/mdp5: don't use void * for opaque types

For example, use 'struct mdp5_smp *' everywhere instead of 'void *', but
only declare it as 'struct mdp5_smp;' in common headers, so the struct
body is still private.  The accomplishes the desired modularity while
still letting the compiler provide some type checking for us.

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