drm/radeon/kms/pm: fix power state indexing on igp chips in dynpm mode

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=28745Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Tested-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/r600.c

@@ -130,10 +130,15 @@ void r600_pm_get_dynpm_state(struct radeon_device *rdev)
 							break;
 						}
 					}
-				} else
+				} else {
+					if (rdev->pm.current_power_state_index == 0)
+						rdev->pm.requested_power_state_index =
+							rdev->pm.num_power_states - 1;
+					else
 						rdev->pm.requested_power_state_index =
 							rdev->pm.current_power_state_index - 1;
 				}
+			}
 			rdev->pm.requested_clock_mode_index = 0;
 			/* don't use the power state if crtcs are active and no display flag is set */
 			if ((rdev->pm.active_crtc_count > 0) &&