drm/xe/guc: Move GuC registers to regs/

There's no good reason to keep the GuC registers outside the regs/
directory: move the header with GuC registers under that.

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