drm/msm/sde: correct crtc client type for writeback

CRTC incorrectly returns RT_RSC_CLIENT client type
for writeback interface. Correct the client type
for writeback as NRT_CLIENT.

Change-Id: Ic8c1d1d96ea609540df1f5ff47568dfcb9bea5fe
Signed-off-by: Alan Kwong <akwong@codeaurora.org>