drm: Move legacy buffer structures to <drm/drm_legacy.h>

A few odd cases:
- mgag200 someho had a totally unused drm_dma_handle_t. Remove it.
- i915 still uses the legacy pci dma alloc api, so grows an include.

Everything else fairly standard.

v2: Include "drm_legacy.h" in drm.ko source files for consistency.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_pci.c

@@ -27,6 +27,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/export.h>
 #include <drm/drmP.h>
+#include "drm_legacy.h"
 
 /**
  * drm_pci_alloc - Allocate a PCI consistent memory block, for DMA.