drm/xe/uapi: Add examples of user space code

Complete the documentation of some structs by adding functional
examples of user space code. Those examples are intentionally kept
very simple. Put together, they provide a foundation for a minimal
application that executes a job using the Xe driver.

v2: Remove use of DRM_XE_VM_BIND_FLAG_ASYNC (Francois Dugast)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Mateusz Naklicki <mateusz.naklicki@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>