drm/vmwgfx: Add back ->detect() and ->fill_modes()

This partially reverts commit d56f57ac ("drm/gma500: Move to private
save/restore hooks") which removed these lines by mistake.

Reported-by: Sebastian Herbszt <herbszt@gmx.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thomas Hellstrom <thellstrom@vmware.com>
Tested-by: Sebastian Herbszt <herbszt@gmx.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>