winsys/svga: Rework buffer allocation to make it more robust v2.

winsys/svga: Rework buffer allocation to make it more robust v2.

Don't allow any "CPU" buffers to be allocated by the pb_fenced
buffer manager, since we can't protect against failures during
buffer validation.
Also, add an extra slab buffer manager to allocate buffers from
the kernel if there is a failure to allocate from our big buffer pool.
The reason we use a slab manager for this, is to avoid allocating
many very small buffers from the kernel.

v2: Increased VMW_MAX_BUFFER_SIZE and fixed some comments.

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