drm/xe/rtp: Add FIELD_SET_FUNC RTP action

drm/xe/rtp: Add FIELD_SET_FUNC RTP action

Most of our RTP programming involves programming constant values into
register fields.  However there are a few cases (e.g., RING_CMD_CCTL
programming) that rely on dynamic per-GT or per-engine checks to decide
what value will be programmed.  Add a FIELD_SET_FUNC RTP action which
will call the provided function pointer once at RTP processing time to
determine the appropriate value.

v2:
 - Tweak kerneldoc to avoid duplicating explanation from FIELD_SET.
   (Gustavo)

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patch.msgid.link/20260617-rtp_with_dynamic_vals-v2-2-3f4cb34c2ea1@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>