accel/amdxdna: Fix mm_struct reference leak in aie2_populate_range()

aie2_populate_range() jumps back to the again label without calling
mmput(mm), leaking a reference to the mm_struct.

Add the missing mmput() before jumping to again.

Fixes: e486147c912f ("accel/amdxdna: Add BO import and export")
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20260610151127.2994185-1-lizhi.hou@amd.com