msm: mdss: Release smp's held for writeback mixers

Smp's reserved for writeback interface can be released safely
as long as the pipeline is idle. This allows pipe reprogramming
during concurrent line and block mode operations.

Change-Id: Ib0fdf4d45412daa4478ac0fb7aff2e3564b2a584
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>