drm/i915: Store dbuf slice mask in device info

Let's just store the dbuf slice information as a bitmask
in the device info. Makes life a little easier later.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416171011.19012-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>