drm/amdgpu: remove redundant memset

kvmalloc_array uses __GFP_ZERO flag ensures that the returned address
is zeroed already, memset it to zero again afterwards is unnecessary,
and in this case buggy because we only clear the first entry.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>