drm/i915: Use intel_dbuf_slice_size()

Use intel_dbuf_slice_size() instead of hand rolling it.
Also clean up some of the types.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416171011.19012-5-ville.syrjala@linux.intel.com


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