drm/vmwgfx: Fix compat shader namespace

Contrary to the host-backed shader interface that has a per-context
name-space for shaders, the compat shader namespace was per client
(or rather, per file). Fix this so that the compat shader namespace is per
context, and at the same time, make command buffer managed context resource
management generic.

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