drm/msm/sde: expose max layers and fix input rect reading for dim_layer

Add debug messages and also fix the input parameter reading of
the dim layer rectangle. And fix the typo, to assign the x-offset
instead of width while setting the split dim layer. The change
also exposes the max supported dim layers through a property.

Change-Id: I2f3a6829eace9563d0f7e9032b0ac24de0f03542
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>