drm/msm/sde: fix incorrect layer mixer count

Not all the SDE layer mixer HW block offsets are valid for a given
target device. All the valid mixer HW block id's need not be
contiguous for a device. This change populates the offset value
as 0 for invalid layer mixer blocks in a non-contiguous scenario.
This helps SDE HW catalog to keep track of actual LM count.

Change-Id: Idd6ec93091355c9c330b042ee0ce847ed4914ac4
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>