disp: msm: sde: add changes to allocate compatible cwb mixers in RM

This change parses new dt property CWB_MIXER_MASK, which signifies
the compatible mixer_mask for the corresponding CWB block.
During mixer allocation for CWB usecase, we use this cwb_mixer_mask
to decide compatible mixers for built-in primary and secondary displays.

In the current issue case, mixer allocation is as below in
multi display usecase:

primary: LM0
secondary: LM1
external: LM2

RM loops through available mixers and tries to allocate LM3 when
CWB is triggered on primary display. But from HW perspective,
LM0 is not muxed to LM3 causing wb timeouts. With current change
LM3 gets skipped and LM4 gets allocated.

Change-Id: I95ce16a083c9b9976a9dff309d7754085ee08958
Signed-off-by: Jayaprakash Madisetty <jmadiset@codeaurora.org>