drm/msm/sde: fix excl_rect overlap validation

Allow layer with exclusion rect, if there is an overlapping
layer above or below, covering the entire exclusion
rect region. Add more information for failure cases to help
in debugging.

Change-Id: If1b4eff8f99db354e6fc08180f6ac06adfe43705
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>