drm/vmwgfx: Use a validation context allocator for relocations and validations

A common trait of these objects are that they are allocated during the
command validation phase and freed after command submission. Furthermore
they are accessed by a single thread only. So provide a simple unprotected
stack-like allocator from which these objects can be allocated. Their
memory is freed with the validation context when the command submission
is done.

Note that the mm subsystem maintains a per-cpu cache of single pages to
make single page allocation and freeing efficient.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>