drm/i915: Consolidate checks for engine stats availability

Sagar noticed the check can be consolidated between the engine stats
implementation and the PMU.

My first choice was a static inline helper but that got into include
ordering mess quickly fast so I went with a macro instead. At some point
we should perhaps looking into taking out the non-ringubffer bits from
intel_ringbuffer.h into a new intel_engine.h or something.

v2: Use engine->flags. (Chris Wilson)
v3: Rebase and mark GuC as not yet supported. (Chris Wilson)
v4: Move flag setting to intel_engines_reset_default_submission.
    (Chris Wilson)
v5: Move flag setting to logical_ring_setup.
v6: intel_engines_reset_default_submission is the wrong place to set the
    flag - it needs to be in execlists_set_default_submission. (Sagar)
v7: Flag setting in logical_ring_setup is not required. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com> (v6)
Link: https://patchwork.freedesktop.org/patch/msgid/20171129102805.22690-1-tvrtko.ursulin@linux.intel.com
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 5708645..2a8160f 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1919,6 +1919,8 @@ static void execlists_set_default_submission(struct intel_engine_cs *engine)
 
 	engine->park = NULL;
 	engine->unpark = NULL;
+
+	engine->flags |= I915_ENGINE_SUPPORTS_STATS;
 }
 
 static void