drm/i915: move modesetting core code under display/

Now that we have a new subdirectory for display code, continue by moving
modesetting core code.

display/intel_frontbuffer.h sticks out like a sore thumb, otherwise this
is, again, a surprisingly clean operation.

v2:
- don't move intel_sideband.[ch] (Ville)
- use tabs for Makefile file lists and sort them

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613084416.6794-3-jani.nikula@intel.com
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index b03e2a4..d9a7a13 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -33,13 +33,14 @@
 #include <drm/drm_fourcc.h>
 #include <drm/drm_plane_helper.h>
 
+#include "display/intel_atomic.h"
+#include "display/intel_fbc.h"
+#include "display/intel_sprite.h"
+
 #include "i915_drv.h"
 #include "i915_irq.h"
-#include "intel_atomic.h"
 #include "intel_drv.h"
-#include "intel_fbc.h"
 #include "intel_pm.h"
-#include "intel_sprite.h"
 #include "intel_sideband.h"
 #include "../../../platform/x86/intel_ips.h"