drm/v3d: Enable BO mapping

For the indirect CSD CPU job, we will need to access the internal
contents of the BO with the dispatch parameters. Therefore, create
methods to allow the mapping and unmapping of the BO.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231130164420.932823-13-mcanal@igalia.com