drm/amdgpu: Fix oops when pp_funcs is unset in ACPI event

commit 5932d260

 upstream.

On ARCTURUS and RENOIR, powerplay is not supported yet.
When plug in or unplug power jack, ACPI event will issue.
Then kernel NULL pointer BUG will be triggered.
Check for NULL pointers before calling.

Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>