decoder: Move initialization of dbp_mgr entries to init_decoder()

Earlier these were only initialized during static buffer allocations.
Initializing them in init_decoder() will ensure that these get
initialized to default values during reset() as well. Without this,
in some error cases, there is a possibility of heap-use-after free,
when resolution changes and these pointers point to memory that is freed

Bug: 142602711
Test: poc in bug
Change-Id: Ie39fee0eca56bf32cdc558099bf167d05eb89620