drm/vmwgfx: Fix a circular locking dependency in the fbdev code

When a user-space process writes directly to the fbdev framebuffer,
we hit a circular locking dependency. Fix this by introducing a local
delayed work callback so that the defio lock can be released before
calling into the modesetting code for a dirty update.

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