drm/vmwgfx: Print message when command verifier returns with error

Whenever command verifier function returns with an error, print a debug
message using VMW_DEBUG_USER. This will make sure failing commands can
be easily tracked for debugging purpose.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>