drm/etnaviv: Ignore MC when checking runtime suspend idleness

Without that runtime suspend is often blocked due to
etnaviv_gpu_rpm_suspend() returning -EBUSY since the FE seems to trigger
the MC in its idle loop.

Ignoring the MC bit makes the GPU suspend as expected. This was tested
on GC7000.

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