drm/panfrost: Hold runtime PM reference until jobs complete

Doing a pm_runtime_put as soon as a job is submitted is wrong as it should
not happen until the job completes. It works currently because we are
relying on the autosuspend timeout to keep the h/w enabled.

Fixes: f3ba9122 ("drm/panfrost: Add initial panfrost driver")
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Alyssa Rosenzweig <alyssa.rosenzweig@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/20190826223317.28509-3-robh@kernel.org