drm/i915: Fix l3 parity user buffer offset

The buf pointer used during l3_write is just char *, therefore it does
not require the silly any addition of offset.

v2: Also fix i915_l3_read with a suggested logic from Ville

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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>