drm/i915: Document the split in internal and public execbuf flags

Since we reuse the same field for the user passing in their control
flags, and for the kernel to track a couple of bits of state, document
and check that those do not overlap.
Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170921110135.15990-1-chris@chris-wilson.co.ukReviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -58,6 +58,7 @@ enum {
 
 #define __EXEC_HAS_RELOC	BIT(31)
 #define __EXEC_VALIDATED	BIT(30)
+#define __EXEC_INTERNAL_FLAGS	(~0u << 30)
 #define UPDATE			PIN_OFFSET_FIXED
 
 #define BATCH_OFFSET_BIAS (256*1024)
@@ -2185,6 +2186,7 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 	int out_fence_fd = -1;
 	int err;
 
+	BUILD_BUG_ON(__EXEC_INTERNAL_FLAGS & ~__I915_EXEC_ILLEGAL_FLAGS);
 	BUILD_BUG_ON(__EXEC_OBJECT_INTERNAL_FLAGS &
 		     ~__EXEC_OBJECT_UNKNOWN_FLAGS);