vc4/resource: Support offset query for multi-planar planes

vc4/resource: Support offset query for multi-planar planes

This is required in order to return the correct value for
`gbm_dri_bo_get_offset()` for e.g. the second plane of a NV12 image.

Use the newly introduced `util_resource` helper and, while on it, also
add support for `gbm_bo_get_plane_count()`.

Cc: mesa-stable
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26283>
(cherry picked from commit cb8cdab928ff2302bef00b341914168c78a89a7c)