drm/amd/display: fix array-bounds error in dc_stream_remove_writeback() [take 2]

drm/amd/display: fix array-bounds error in dc_stream_remove_writeback() [take 2]

Commit 5d8c3e836fc2 ("drm/amd/display: fix array-bounds error in
dc_stream_remove_writeback()") tried to fix an array bounds error seen
with gcc 12.0. Unfortunately, that results in another array bounds error,
seen with older versions of gcc.

Building csky:allmodconfig ... failed
--------------
Error log:
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:
	In function 'dc_stream_remove_writeback':
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:527:83:
	error: array subscript 1 is above array bounds of 'struct dc_writeback_info[1]' [-Werror=array-bounds]
  527 |                                 stream->writeback_info[j] = stream->writeback_info[i];
      |                                                             ~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from drivers/gpu/drm/amd/amdgpu/../display/dc/dc.h:1269,
                 from drivers/gpu/drm/amd/amdgpu/../display/dc/inc/core_types.h:29,
                 from drivers/gpu/drm/amd/amdgpu/../display/dc/basics/dc_common.h:29,
                 from drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:27:
drivers/gpu/drm/amd/amdgpu/../display/dc/dc_stream.h:241:34: note: while referencing 'writeback_info'
  241 |         struct dc_writeback_info writeback_info[MAX_DWB_PIPES];

We could check both i and j for overflow to fix the problem. That would,
however, be not make much sense since it is known and provable that j <= i.
Also, the check introduced with commit 5d8c3e836fc2 does not really add
value since it checks if j < MAX_DWB_PIPES. Since it is known that j <= i,
it would make more sense to check if i < MAX_DWB_PIPES. Unfortunately, that
does not help to solve the problem observed here: gcc still complains.

To solve the problem, replace the subsequent check for 'i != j' with
'j < i'. This is identical to the original check since we know that j <= i,
and it makes all versions of gcc happy. Drop the check introduced with
commit 5d8c3e836fc2 since it is not really useful and does not solve the
problem.

Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Fixes: 5d8c3e836fc2 ("drm/amd/display: fix array-bounds error in dc_stream_remove_writeback()")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>