drm/xe/rtp: Add support for entries with no action

Add a separate struct to hold entries in a table that has no action
associated with each of them. The goal is that the caller in future can
set a per-context callback, or just use the active entry marking
feature.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230526164358.86393-11-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>