drm/msm/mdp5: Prepare Layer Mixers for source split

In order to enable Source Split in HW, we need to add/modify
a few LM register configurations:

- Configure the LM width to be half the mode width, so that
  each LM manages one half of the scanout.
- Tell the 'right' LM that it is configured to be the 'right'
  LM in source split mode.
- Since we now have 2 places where REG_MDP5_LM_BLEND_COLOR_OUT is
  configured, do a read-update-store for the register instead of
  directly writing a value to it.

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