drm/i915: Round l3 parity reads down

We always read a register for l3 parity reads, and we don't really want
to ever let userspace trick us into giving back less than the dword.

Writes are okay because we assume everything will be 0 filled, and as
such, if a user really wants to write less than a dword, let them.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>