drm/radeon: fix semaphore value init

Semaphore values have 64 bits, not 32. This fixes a very subtle bug
that disables synchronization when the upper 32bits wasn't zero.

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Reviewed-By: Grigori Goronzy <greg@chown.ath.cx>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>