drm/i915: Fix bits vs. bytes mixup in dbuf block size computation

drm/i915: Fix bits vs. bytes mixup in dbuf block size computation

The spec used to say "8bpp" which someone took to mean 8 bytes per
pixel when in fact it was supposed to be 8 bits per pixel. The
spec has been updated to make it more clear now. Fix the code
to match.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181221171436.8218-4-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>