drm/amd/display: use do-while-0 for DC_TRACE_LEVEL_MESSAGE()

Building with W=1 complains about an empty 'else' statement, so use the
usual do-nothing-while-0 loop to quieten this warning.

../drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dmub_psr.c:113:53: warning: suggest braces around empty body in an 'else' statement [-Wempty-body]
  113 |                                 *state, retry_count);

Fixes: b30eda8d ("drm/amd/display: Add ETW log to dmub_psr_get_state")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Wyatt Wood <wyatt.wood@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce/dmub_psr.c

@@ -29,7 +29,7 @@
 #include "dmub/dmub_srv.h"
 #include "core_types.h"
 
-#define DC_TRACE_LEVEL_MESSAGE(...) /* do nothing */
+#define DC_TRACE_LEVEL_MESSAGE(...)	do {} while (0) /* do nothing */
 
 #define MAX_PIPES 6