drm/msm/sde: add CWB custom property support

If concurrent writeback feature is supported by HW, add a
new property on each CRTC through which client can request
the data point of capture (LM or DSSP output). Availability
of this property on crtc itself is indication for client about
the availability of CWB feature.

Change-Id: Ia6cf3b3f544d8d2f71fc2bcd376081f629e9d8ba
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>