mmc: core: Update cmdq for making use of per-request data

Update cmdq initialization path for making use of per-request data.
Also, update cmdq request processing paths per this new change.

Since we are making use of per-request data, no need to maintain
the statically allocated array of mmc_queue_req data structure.

Change-Id: I65fa9fbb45c97835115405d558db764e6189d5f3
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>