drm/v3d: Create a struct to store the GPU stats

This will make it easier to instantiate the GPU stats variables and it
will create a structure where we can store all the variables that refer
to GPU stats.

Note that, when we created the struct `v3d_stats`, we renamed
`jobs_sent` to `jobs_completed`. This better express the semantics of
the variable, as we are only accounting jobs that have been completed.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240420213632.339941-4-mcanal@igalia.com