drm/amdgpu: remove memset after kzalloc

kzalloc has already zeroed the memory during the allocation.
So memset is unneeded.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>