drm/amdgpu: remove not needed memset

Fixes coccicheck warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_ih.c:64:13-31: WARNING: dma_alloc_coherent use in ih -> ring already zeroes out memory,  so memset is not needed

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>