drm/i915: Make vblank evade warnings optional

Add a new Kconfig option to enable/disable the extra warnings
from the vblank evade code. For now we'll keep the warning
about an actually missed vblank always enabled as that can have
an actual user visible impact. But if we miss the deadline
othrwise there's no real need to bother the user with that.
We'll want these warnings enabled during development however
so that we can catch regressions.

Based on the reports it looks like this is still very easy
to hit on SKL, so we have more work ahead of us to optimize
the crtiical section further.

Cc: Daniel Vetter <[e-mail]>
Cc: Jani Nikula <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Reported-by: Jens Axboe <[e-mail]>
Reported-by: Linus Torvalds <[e-mail]>
Fixes: e1edbd44e23b ("drm/i915: Complain if we take too long under vblank evasion.")
Signed-off-by: Ville Syrjälä <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/i915/Kconfig.debug

···
           and also analyze the request dependency resolving timeline.
 
           If in doubt, say "N".
+
+config DRM_I915_DEBUG_VBLANK_EVADE
+	bool "Enable extra debug warnings for vblank evasion"
+	depends on DRM_I915
+	default n
+	help
+	  Choose this option to turn on extra debug warnings for the
+	  vblank evade mechanism. This gives a warning every time the
+	  the deadline allotted for the vblank evade critical section
+	  is exceeded, even if there isn't an actual risk of missing
+	  the vblank.
+
+	  If in doubt, say "N".

drivers/gpu/drm/i915/intel_sprite.c

···
 			  ktime_us_delta(end_vbl_time, crtc->debug.start_vbl_time),
 			  crtc->debug.min_vbl, crtc->debug.max_vbl,
 			  crtc->debug.scanline_start, scanline_end);
-	} else if (ktime_us_delta(end_vbl_time, crtc->debug.start_vbl_time) >
-		   VBLANK_EVASION_TIME_US)
+	}
+#ifdef CONFIG_DRM_I915_DEBUG_VBLANK_EVADE
+	else if (ktime_us_delta(end_vbl_time, crtc->debug.start_vbl_time) >
+		 VBLANK_EVASION_TIME_US)
 		DRM_WARN("Atomic update on pipe (%c) took %lld us, max time under evasion is %u us\n",
 			 pipe_name(pipe),
 			 ktime_us_delta(end_vbl_time, crtc->debug.start_vbl_time),
 			 VBLANK_EVASION_TIME_US);
+#endif
 }
 
 static void