mmc: add dynamic trigger for packed control

In the current implementation packing is enabled according to a statically
defined trigger. This patch updates the packing control mechanism to use a
dynamically defined trigger.
The trigger's value is calculated by the relation
between the number of potential packed write requests and the mean
value of all previous potential values:
If the current potential is greater than the mean potential then
the heuristic is that the following workload will contain many write
requests, therefore we lower the packed trigger. In the opposite case
we want to increase the trigger in order to get less packing events.
In case we get an urgent request we 'punish' the packing control by
increasing the trigger.

Change-Id: I4fba6096f7be02b8f691c1d49d9b64f01d3c37e4
Signed-off-by: Lee Susman <lsusman@codeaurora.org>