drm/xe: Add MI_LOAD_REGISTER_REG command definition

The MI_LOAD_REGISTER_REG command reads value from a source register
location and writes that value to a destination register location.

Bspec: 45730, 60233
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250303173522.1822-2-michal.wajdeczko@intel.com