drm/vmwgfx: Fixes to vmwgfx_fb

1.  When unsetting a mode, num_connector should be set to zero
2.  The pixel_format field needs to be initialized as newer DRM internal
    functions checks this field
3.  Take the drm_modeset_lock_all() because vmw_fb_kms_detach() can
    change current mode

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