msm: mdss: fix possible deadlock while cleaning buffers

Move pipe destroy logic outside of mfd lock to fix lockdep warning
regarding pipe unlock logic being called within mfd lock, when mfd lock
is usually locked within pipe lock and remove SSPP lock when destroying
pipes. This was previously used to protect SMP allocations but these are
protected by SMP lock already.

Change-Id: I1c47da5d33cb4f1eb687078639adf4c4cc94f33a
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>