msm: mdss: Fix DSPP usage inconsistency issue

Dither and IGC blocks in DSPP HW are getting enabled
irrespective of left and right mixers being attached
to DSPP blocks. Ensure that DSPP Dither and IGC blocks
are enabled only when ctrl path either has single mixer
or both mixers are attached to DSPP.
Disable default IGC and Dither configuration on ctrl
destroy.

Change-Id: I98d1b356856886cef296e6e4d300fc2dd9cad314
CRs-Fixed: 2001867
Signed-off-by: Rajesh Yadav <ryadav@codeaurora.org>