drm/msm/sde: add special handling for post-proc BL properties

The post-processing clients are using legacy set property API
for two BL scale properties in connector, which doesn't fit
into the atomic set property framework.
This change adds special handling for these two non-atomic
BL properties.

CRs-fixed: 2144594
Change-Id: I3737e97c890b0ed635bd9c5e8284b747c622aef7
Signed-off-by: Xu Yang <yangxu@codeaurora.org>