drm: Extract drm_dumb_buffers.c

Just code movement, doc cleanup will follow up later.

v2: Keep all the copyright notices.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114115825.22050-2-daniel.vetter@ffwll.ch
diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index f217274..adcfc8f 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -15,7 +15,8 @@
 		drm_modeset_lock.o drm_atomic.o drm_bridge.o \
 		drm_framebuffer.o drm_connector.o drm_blend.o \
 		drm_encoder.o drm_mode_object.o drm_property.o \
-		drm_plane.o drm_color_mgmt.o drm_print.o
+		drm_plane.o drm_color_mgmt.o drm_print.o \
+		drm_dumb_buffers.o
 
 drm-$(CONFIG_COMPAT) += drm_ioc32.o
 drm-$(CONFIG_DRM_GEM_CMA_HELPER) += drm_gem_cma_helper.o