radeonsi: fix user fence space when MCBP is enabled
When MCBP is enabled, IB maybe preempted which will also update the preempted fence field of the user fence. So we need to reserve enough space for each user fence.
Fixes: 89d2dac5 "radeonsi: enable preemption if the kernel enabled it" Signed-off-by: Qiang Yu yuq825@gmail.com