drm/xe: Rename RC0/RC6 macros

Follow up commits will mass-remove the gen prefix/suffix. For GEN6_RC0
and GEN6_RC6 that would make the variable too short and easy to
conflict. So, add "GT_" prefix that is also part of the register name.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>