drm/msm/mdp5: subclass CRTC state

Subclass drm_crtc_state so that we can maintain additional state for
our CRTCs.

Add mdp5_pipeline and mdp5_ctl pointers in the subclassed state.
mdp5_pipeline is a grouping of the HW entities that forms the downstream
pipeline for a particular CRTC. It currently contains pointers to
mdp5_interface and mdp5_hw_mixer tied to this CRTC. Later, we will
have 2 hwmixers in this struct. (We could also have 2 intfs if we want
to support dual DSI with Source Split enabled. Implementing that feature
isn't planned at the moment).

The mdp5_pipeline state isn't used at the moment. For now, we just
introduce mdp5_crtc_state and the crtc funcs needed to manage the
subclassed state.

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