drm/msm/mdp5: describe LM instances in mdp5_cfg

The number of Layer Mixers and the downstream blocks (DSPPs and PPs)
connected to each LM can vary with different MDP5 revisions. These
parameters are also static.

Keep the per instance LM data in mdp5_cfg. This will avoid the need
to have macros which identify PP id or DSPP id the LM is connected
to. We don't configure DSPPs at the moment, but keeping the DSPP
instance # here might come handy later.

Also add a 'caps' field that identifies features supported by a
LM instance. Introduce the caps MDP_LM_CAP_DISPLAY and MDP_LM_CAP_WB
that identify whether a LM instance can be used for display or
writeback.

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