Currently, we manually perform all operations to update the GPU stats
variables. Apart from the code repetition, this is very prone to errors,
as we can see on commit 35f4f8c9 ("drm/v3d: Don't increment
`enabled_ns` twice").

Therefore, create two functions to manage updating all GPU stats
variables. Now, the jobs only need to call for `v3d_job_update_stats()`
when the job is done and `v3d_job_start_stats()` when starting the job.
Co-developed-by: Tvrtko Ursulin <tursulin@igalia.com>
Signed-off-by: Tvrtko Ursulin <tursulin@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240420213632.339941-3-mcanal@igalia.com