drm/i915: Add intel_context tracing

Add intel_context tracing. These trace points are particular helpful
when debugging the GuC firmware and can be enabled via
CONFIG_DRM_I915_LOW_LEVEL_TRACEPOINTS kernel config option.

Cc: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721215101.139794-19-matthew.brost@intel.com