drm/amd/display: Fix duplicate included clk_mgr.h

Clean up the following includecheck warning:

./drivers/gpu/drm/amd/display/dc/dcn31/dcn31_hwseq.c: clk_mgr.h is
included more than once.

No functional change.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn31/dcn31_hwseq.c

@@ -33,7 +33,6 @@
 #include "clk_mgr.h"
 #include "reg_helper.h"
 #include "abm.h"
-#include "clk_mgr.h"
 #include "hubp.h"
 #include "dchubbub.h"
 #include "timing_generator.h"