drm/i915/tracepoints: Add request submit and execute tracepoints

These new tracepoints are emitted once the request is ready to
be submitted to the GPU and once the request is about to
be submitted to the GPU, respectively.

Former condition triggers as soon as all the fences and
dependencies have been resolved, and the latter once the
backend is about to submit it to the GPU.

New tracepoint are enabled via the new
DRM_I915_LOW_LEVEL_TRACEPOINTS Kconfig option which is disabled
by default to alleviate the performance impact concerns.

v2: Move execute tracepoint to __i915_gem_request_submit.
    (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>