drm/amd: use list_for_each_entry for list iteration.

list_for_each() can be replaced by the more concise
list_for_each_entry() here for iteration over the lists.
This change was reported by coccinelle.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>