msm: mdss: fix req availability check in dsi command commit

NULL check for req in dsi cmd list commit is misplaced, causing
race condition due to skipping of dsi cmd mdp busy check. This would
eventually result in underflow due to possible contention of using dsi
link. Fix the NULL check to avoid misbehavior.

Change-Id: I5999646d4db67efd0854b3db07e1d987e66c9cb5
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>