mmc: block: serialize the requests if we are scaled down

When in SVS2 on low load scenario and there are lots of requests
queued for CMDQ we need to wait till the queue is empty to scale
back up to Nominal even if there is a sudden increase in load.
This impacts performance where lots of IO get executed in SVS2
frequency since the queue is full. As SVS2 is a low load use case
we can serialize the requests and not queue them in parallel
without impacting other use cases. This makes sure the queue gets
empty faster and we will be able to scale up to Nominal frequency
when needed.

Change-Id: Idbe7e939e01327061dfa5de93c0eaed59b910592
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>