drm/panfrost: Make sure GPU is powered on when reading GPU_LATEST_FLUSH_ID

Bifrost devices do support the flush reduction feature, so on first job
submit we were trying to read the register while still powered off.

If the GPU is powered off, the feature doesn't bring any benefit, so
don't try to read.

Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200611085900.49740-1-tomeu.vizoso@collabora.com