drm/xe: Add helper macro to loop each DSS

Add helper macro to loop each DSS. This is a precursor patch to allow
for easier iteration through MCR registers and other per-DSS uses.

Signed-off-by: Zhanjun Dong <zhanjun.dong@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240314210735.258553-2-zhanjun.dong@intel.com