Two new tracepoints placed at the call sites where requests are
actually passed to the GPU enable userspace to track engine
utilisation.

These tracepoints are only enabled when the
DRM_I915_LOW_LEVEL_TRACEPOINTS Kconfig option is enabled.

v2: Fix compilation with !CONFIG_DRM_I915_LOW_LEVEL_TRACEPOINTS.

v3: Name global seqno consistently across tracepoints.

v4: Remove port info from request out tracepoint. (Chris Wilson)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>