libdrm: amdgpu: Initialize unions with memset rather than "= {0}"

Clang complains when initializing unions using "= {0}"
so instead use memset.

Cc: Sean Paul <seanpaul@chromium.org>
Cc: Alistair Strachan <astrachan@google.com>
Cc: Marissa Wall <marissaw@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>