drm/msm/sde: fix excl_rect validation in crtc

u16 flag is passed incorrectly while populating the
plane's src rect to compare against excl_rect. Fix it
to pass the correct flag.

Change-Id: Icc8686e4fe4644b1a7d7b479af211c29bec927bd
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>