drm/i915: Introduce intel_dbuf_slice_size()

Put the code into a function with a descriptive name. Also relocate
the code a bit help future work.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122205633.18492-4-ville.syrjala@linux.intel.com