drm/vmwgfx: Fix false lockdep warning

A lockdep warning is hit when evicting surfaces and reserving the backup
buffer. Since this buffer can only be reserved by the process holding the
surface reservation or by the buffer eviction processes that use tryreserve,
there is no real deadlock here, but there's no other way to silence lockdep
than to use a tryreserve. This means the reservation might fail if the buffer
is about to be evicted or swapped out, but we now have code in place to
handle that reasonably well.

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