drm/radeon/kms/atom: fix possible segfault in pm setup

If we end up with no power states, don't look up
current vddc.

fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=44130

agd5f: fix patch formatting
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_atombios.c

@@ -2560,7 +2560,11 @@ void radeon_atombios_get_power_modes(struct radeon_device *rdev)
 
 	rdev->pm.current_power_state_index = rdev->pm.default_power_state_index;
 	rdev->pm.current_clock_mode_index = 0;
-	rdev->pm.current_vddc = rdev->pm.power_state[rdev->pm.default_power_state_index].clock_info[0].voltage.voltage;
+	if (rdev->pm.default_power_state_index >= 0)
+		rdev->pm.current_vddc =
+			rdev->pm.power_state[rdev->pm.default_power_state_index].clock_info[0].voltage.voltage;
+	else
+		rdev->pm.current_vddc = 0;
 }
 
 void radeon_atom_set_clock_gating(struct radeon_device *rdev, int enable)