drm/msm: use local non-blocking flag to avoid use after free

Avoid checking nonblock flag in the commit packet that gets
dispatched to a worker thread, since that worker thread may free
the commit packet before the dispatch thread has a chance to
read it again after the commit dispatch loop.

Change-Id: Idcbc17dd1cb6df15d0f9607b589587d6f03ee4ff
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>