drm/etnaviv: always allocate 4K for kernel ringbuffers

Since the kernel ringbuffers are allocated from a larger suballocated
area, same as the user commandbufs, they don't need to be CPU page
sized. Allocate 4KB for the kernel ring buffers, as we never use more
than that.

Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>