drm/amdgpu: fix mc_data out-of-bounds read warning

[ Upstream commit 51dfc0a4d609fe700750a62f41447f01b8c9ea50 ]

Clear warning that read mc_data[i-1] may out-of-bounds.

Signed-off-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 5fa4df25ecfc7b6c9006f5b871c46cfe25ea8826)
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>