drm/i915: Use local variables for subslice_mask for device info

When setting up subslice_mask, instead of operating on the slice
array directly, use a local variable to start bits per slice, then
use this to set the per slice array in one step.

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823160307.180813-6-stuart.summers@intel.com