disp: msm: sde: fix crtc client type checks for WB

CRTC client type is wrongly identified as RT_CLIENT
for WB as the API considers only RT and RT_RSC. Resolve
it by adding a new API to check for RT CTRC.

Change-Id: I1f216f60a18215426e594d0f8b09852af376799d
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>