drm/i915: Add tracepoints to track a vm during its lifetime

- ppgtt init/release: these tracepoints are useful for observing the
  creation and destruction of Full PPGTTs.

- ctx create/free: we can use the ctx_free trace in combination with the
  ppgtt_release one to be sure that the ppgtt doesn't stay alive for too
  long after the ctx is destroyed. ctx_create is there for simmetry

- switch_mm: important point in the lifetime of the vm

v4: add DOC information
v5: pull the DOC in drm.tmpl
v6: clean ppgtt init/release traces + add ctx create/free and switch_mm
    tracepoints (Chris)
v7: drop execlist_submit_context tracepoint

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>