drm/etnaviv: split out starting of FE idle loop

Move buffer setup and starting of the FE loop in the kernel ringbuffer
into a separate function. This is a preparation to start the FE later
in the submit process.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Guido Günther <agx@sigxcpu.org>