drm/i915: Fix pointer-to-int conversion

Commit faf65486

 ("drm/i915: Unify uC variable types to avoid
flooding checkpatch.pl") breaks 32-bit kernel builds. Lets use
cast helper to make compiler happy.

v2: introduce ptr_to_u64 (Chris)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171006130844.49012-1-michal.wajdeczko@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>