drm/msm: Fix the check for the command size

The overrun check for the size of submitted commands is off by one.
It should allow the offset plus the size to be equal to the
size of the memory object when the command stream is very tightly
constructed.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>