drm/msm: fix cmdstream size check

Need to check size+offset against bo size (duh!).. now we have a test
case to make sure I've done it right:

https://github.com/freedreno/msmtest/blob/master/submittest.c

Also, use DRM_ERROR() for error case traces, which makes debugging
userspace easier when enabling debug traces is too much.

Signed-off-by: Rob Clark <robdclark@gmail.com>