msm: msm_bus: Allocate commit buffers only once

Instead of continuously allocating and freeing buffers on every
client request, allocate a large enough buffer at init for all
usecases and reuse. This should help in usecases where system is
under memory pressure and also improve on performance.

Change-Id: I74c6f1d7407eb85a16fab67b2dc9323d0062ebbf
Signed-off-by: David Dai <daidavid1@codeaurora.org>