slim_msm: Optimize slimbus data port's connection with BAM pipe

Slimbus data port on apps side can be connected in any diretion (as
source, or as destination). BAM pipe connected to that port should
only be disconnected/reconnected if this port direction changes.
This helps with reducing calls to connect/disconnect BAM pipes.
Allocate descriptors to avoid getting descriptor-space from the
common pool and free them only when the port is deallocated.

Change-Id: Ie31acdb9e4c4bba12f1cc70d0c9ad7bf471b3056
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>