drm/xe: Rename MCFG_MCR_SELECTOR to STEER_SEMAPHORE

drm/xe: Rename MCFG_MCR_SELECTOR to STEER_SEMAPHORE

The register at offset 0xfd0 was incorrectly named MCFG_MCR_SELECTOR,
likely copied from i915. According to the hardware specification (Bspec),
this register is actually called STEER_SEMAPHORE.

Rename the register definition and update its usage in xe_gt_mcr.c to
match the official hardware documentation.

No functional changes.

v2: Add Bspec reference (Tejas)

Bspec: 67113
Signed-off-by: Nitin Gote <nitin.r.gote@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Link: https://lore.kernel.org/r/20250723141039.3848390-1-nitin.r.gote@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>