drm/i915/selftests: make mock_drm.h self-contained

Needs i915_drv.h because i915 gets dereferenced.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191219155652.2666-2-jani.nikula@intel.com

drivers/gpu/drm/i915/selftests/mock_drm.h

@@ -27,7 +27,8 @@
 
 #include <drm/drm_file.h>
 
-struct drm_i915_private;
+#include "i915_drv.h"
+
 struct drm_file;
 struct file;