drm/vmwgfx: Fix uninitialized return in vmw_kms_helper_dirty()

Function vmw_kms_helper_dirty() uses the uninitialized variable ret as
return value. Make the result deterministic and directly return as the
variable is unused anyway. Detected by Coverity CID 1324255.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>