drm: make drm_trace.h self-contained

Do not require users of drm_trace.h to include other
files just to let it build

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20190526173535.32701-7-sam@ravnborg.org

drivers/gpu/drm/drm_trace.h

@@ -6,6 +6,8 @@
 #include <linux/types.h>
 #include <linux/tracepoint.h>
 
+struct drm_file;
+
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM drm
 #define TRACE_INCLUDE_FILE drm_trace