drm/vmwgfx: Add kms helpers for dirty- and readback functions

We need to make the dirty- and readback functions callable without a struct
drm_file pointer. We also need to unify the handling of dirty- and readback
cliprects that are now implemented in various places across the kms system,
som add helpers to facilitate this.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>