cadence_gem: Correct the interupt logic

cadence_gem: Correct the interupt logic

This patch fixes two mistakes in the interrupt logic.

First we only trigger single-queue or multi-queue interrupts if the status
register is set. This logic was already used for non multi-queue interrupts
but it also applies to multi-queue interrupts.

Secondly we need to lower the interrupts if the ISR isn't set. As part
of this we can remove the other interrupt lowering logic and consolidate
it inside gem_update_int_status().

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 438bcc014f8f8a2f8f68f322cb6a53f4c04688c2.1491947224.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>