drm/amdgpu: avoid buffer overflow attach in smu_sys_set_pp_table()

It malicious user provides a small pptable through sysfs and then
a bigger pptable, it may cause buffer overflow attack in function
smu_sys_set_pp_table().

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Jiang Liu <gerry@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org