drm/i915: Fix read back of plane stride register

According to the specifications bit 6 is actually valid in the stride register.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rafael Barbalho <rafael.barbalho@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>