drm/i915/selftest_migrate: Consider the possible roundup of size

Consider the possible round up happened at obj size alignment to
min_page_size during the obj allocation.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220405150840.29351-7-ramalingam.c@intel.com