drm/msm/sde: Add support for selecting LM for CWB

There is no requirement populated for CWB pinpong, it
chooses in first come first serve basis. But there can
be MDSS revision HW which allows only specific pingpong
for CWB. Add support for same to reserve correct LM
for CWB path.

Change-Id: I0f7cd01671112529f069332014072f272245129d
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>