drm/i915/selftests: fixup min_alignment usage

Trying to cast the region id into the region type doesn't work too well,
since the i915_vm_min_alignment() won't give us the correct value for
the stolen-lmem case.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@linux.intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220420181613.70033-3-matthew.auld@intel.com