mmc: switch to blk_mq_alloc_disk

Use the blk_mq_alloc_disk to allocate the request_queue and gendisk
together.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20210616053934.880951-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>