pipebuffer, winsys/svga: Add functionality to update pb_validate_entry flags

pipebuffer, winsys/svga: Add functionality to update pb_validate_entry flags

In order to be able to add access modes to a pb_validate_entry, update
the pb_validate_add_buffer function to take a pointer hash table and also
to return whether the buffer was already on the validate list.

Update the svga winsys accordingly.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>