drm/i915: Read C0DRB3/C1DRB3 as 16 bits again

We've defined C0DRB3/C1DRB3 as 16 bit registers, so access them
as such.

Fixes: 1c8242c3 ("drm/i915: Use unchecked writes for setting up the fences")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421153401.13847-3-ville.syrjala@linux.intel.com