drm/v3d: Use v3d_get_extensions() to parse CPU job data

Currently, v3d_get_extensions() only parses multisync data and assigns
it to the `struct v3d_submit_ext`. But, to implement the CPU job with
user extensions, we want v3d_get_extensions() to be able to parse CPU
job data and assign it to the `struct v3d_cpu_job`.

Therefore, allow the function v3d_get_extensions() to use `struct v3d_cpu_job *`
as a parameter. If the `struct v3d_cpu_job *` is assigned to NULL, it means
that the job is a GPU job and CPU job extensions should be rejected.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231130164420.932823-10-mcanal@igalia.com