drm/msm/mdp5: Fix negative SMP block allocation

In case we request a number of SMP blocks which is lower than
the already reserved blocks, we should not try to allocate a
negative number, but 0 blocks instead.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>