drm/panfrost: Split mmu_hw_do_operation into locked and unlocked version

In preparation to call mmu_hw_do_operation with the as_lock already held,
Add a mmu_hw_do_operation_locked function.

Fixes: 7282f764 ("drm/panfrost: Implement per FD address spaces")
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190826223317.28509-6-robh@kernel.org