dmaengine: qcom-bam: Process multiple pending descriptors

The bam dmaengine has a circular FIFO to which we
add hw descriptors that describes the transaction.
The FIFO has space for about 4096 hw descriptors.

Currently we add one descriptor and wait for it to
complete with interrupt and then add the next pending
descriptor. In this way, the FIFO is underutilized
since only one descriptor is processed at a time, although
there is space in FIFO for the BAM to process more.

Instead keep adding descriptors to FIFO till its full,
that allows BAM to continue to work on the next descriptor
immediately after signalling completion interrupt for the
previous descriptor.

Also when the client has not set the DMA_PREP_INTERRUPT for
a descriptor, then do not configure BAM to trigger a interrupt
upon completion of that descriptor. This way we get a interrupt
only for the descriptor for which DMA_PREP_INTERRUPT was
requested and there signal completion of all the previous completed
descriptors. So we still do callbacks for all requested descriptors,
but just that the number of interrupts are reduced.

CURRENT:

            ------      -------   ---------------
            |DES 0|     |DESC 1|  |DESC 2 + INT |
            ------      -------   ---------------
               |           |            |
               |           |            |
INTERRUPT:   (INT)       (INT)	      (INT)
CALLBACK:     (CB)        (CB)         (CB)

		MTD_SPEEDTEST READ PAGE: 3560 KiB/s
		MTD_SPEEDTEST WRITE PAGE: 2664 KiB/s
		IOZONE READ: 2456 KB/s
		IOZONE WRITE: 1230 KB/s

	bam dma interrupts (after tests): 96508

CHANGE:

        ------  -------    -------------
        |DES 0| |DESC 1   |DESC 2 + INT |
        ------  -------   --------------
				|
				|
          		      (INT)
			      (CB for 0, 1, 2)

		MTD_SPEEDTEST READ PAGE: 3860 KiB/s
		MTD_SPEEDTEST WRITE PAGE: 2837 KiB/s
		IOZONE READ: 2677 KB/s
		IOZONE WRITE: 1308 KB/s

	bam dma interrupts (after tests): 58806

Signed-off-by: Sricharan R <sricharan@codeaurora.org>
Reviewed-by: Andy Gross <andy.gross@linaro.org>
Tested-by: Abhishek Sahu <absahu@codeaurora.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>