drm/i915/gvt: fix bad 32 bit shift in gtt

Since ioread32 returns a 32-bit value, it is impossible to left-shift
this value by 32 bits (it produces a compilation error). Casting the
return value of ioread32 fix this issue.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>