drm/amdkfd: Preserve CP_MQD_IQ_RPTR internal state

CP microcode uses undocumented bits in this register to record queue
state information. The KFD zeroes these bits in update_mqd, when invoked
through the UPDATE_QUEUE ioctl, causing incoherent state when the ioctl
is used to successively unmap and map a queue.

Since the queue type cannot be changed in this path, move the MQD write
to init_mqd.

Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Reviewed-by: Ben Goz <ben.goz@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>