disp: msm: sde: Add check to avoid list double add

When LTM hist disable and LTM queue buffer are both queued to driver
in the same commit, if LTM hist disable call is handled by driver first,
all the LTM hist buffers will be moved to buffer_free list. When LTM
queue buffer call is handled, it will try to add the same buffer
to the buffer_free list again, and cause a list double add error.
This change checks if the buffer is already in the buffer_free list
before adding.

Change-Id: I74debdede551ceacfa2735fdb327a37654ad70f7
Signed-off-by: Ping Li <pingli@codeaurora.org>