drm/panfrost: Stay in the threaded MMU IRQ handler until we've handled all IRQs

Doing a hw-irq -> threaded-irq round-trip is counter-productive, stay
in the threaded irq handler as long as we can.

v2:
* Rework the loop to avoid a goto

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210205111757.585248-4-boris.brezillon@collabora.com