drm/msm: avoid dim layer setup if not required

Dim layer clearing/setup requires reading registers in order to update
value, doing this add additional CPU processing when it's not really
needed. Add logic to only do the updates only when needed.

Bug: 142504774
Change-Id: I23bcbe39575de35c387cfb7d2b9dc993525e4f98
Signed-off-by: Adrian Salido <salidoa@google.com>
Signed-off-by: Samantha Tran <samtran@codeaurora.org>