drm: Include prototype for drm_need_swiotlb()

Sparse is not happy:
../drivers/gpu/drm/drm_memory.c:159:6: warning: symbol 'drm_need_swiotlb' was not declared. Should it be static?

Include the correct header for drm_need_swiotlb() prototype.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710125143.9965-3-ville.syrjala@linux.intel.comReviewed-by: Sean Paul <sean@poorly.run>

drivers/gpu/drm/drm_memory.c

@@ -40,6 +40,7 @@
 #include <xen/xen.h>
 
 #include <drm/drm_agpsupport.h>
+#include <drm/drm_cache.h>
 #include <drm/drm_device.h>
 
 #include "drm_legacy.h"