drm/v3d: Create two functions to update all GPU stats variables

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