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 <[e-mail]>
Acked-by: Daniel Vetter <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: David Airlie <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_trace.h

···
 #include <linux/types.h>
 #include <linux/tracepoint.h>
 
+struct drm_file;
+
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM drm
 #define TRACE_INCLUDE_FILE drm_trace