drm: Move drm_clflush prototypes to drm_cache header file

Continue to clean up drmP.h by moving the cache flushing functions into
it's own header file.

Compile-tested only

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109215649.6860-2-krisman@collabora.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -49,6 +49,7 @@
 #include <drm/drm_legacy.h> /* for struct drm_dma_handle */
 #include <drm/drm_gem.h>
 #include <drm/drm_auth.h>
+#include <drm/drm_cache.h>
 
 #include "i915_params.h"
 #include "i915_reg.h"