vmwgfx: Fix confusion caused by using "fence" in various places

This is needed before we introduce the fence objects.
Otherwise this will be even more confusing. The plan is to use the following:

seqno: A 32-bit sequence number that may be passed in the fifo.
marker: Objects, carrying a seqno, that track fifo submission time. They
are used for fifo lag based throttling.
fence objects: Kernel space objects, possibly accessible from user-space and
carrying a 32-bit seqno together with signaled status.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>