drm/xe/mcr: Make xe_gt_mcr_get_dss_steering() input gt a const

drm/xe/mcr: Make xe_gt_mcr_get_dss_steering() input gt a const

Make gt, input parameter to xe_gt_mcr_get_dss_steering(), a
constant. This would allow xe_gt_mcr_get_dss_steering() to
be called from functions that have gt as const to struct xe_gt.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Link: https://lore.kernel.org/r/9dc621a90880f62ac8e2951afea7952277f7eb0e.1756855529.git.harish.chegondi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>