drm/vmwgfx: Remove a few unused functions

I noticed that there is a prototype for vmw_fifo_ping_host_locked() but
no function. Then I looked further and noticed more functions which are
not used anymore or functions protoypes which remained after the
function was removed.

Remove unused function (prototypes).

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>