drm/amd/pm: fix memleak of dpm_policies on smu v15

drm/amd/pm: fix memleak of dpm_policies on smu v15

In smu_v15_0_fini_smc_tables, dpm_policies was not freed or NULLed, causing a memory leak.
Add kfree() and NULL assignment to properly release memory and avoid dangling pointers.

Fixes: 2beedc3a92b7 ("drm/amd/pm: Add initial support for smu v15_0_8");
Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>