drm/msm/sde: fix dim_layer stage validation against planes

Currently in atomic_check, dim_layer stages are checked against
pipe stages without accounting for STAGE_0 offset. Add the offset
for proper validation.

Change-Id: Id867d3cc64871dfb7e7404dda263319a824d85b3
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>