drm/v3d: Decouple stats calculation from printing

Create a function to decouple the stats calculation from the printing.
This will be useful in the next step when we add a seqcount to protect
the stats.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240420213632.339941-6-mcanal@igalia.com