drm/radeon: enable native backlight control on old macs

Commit b7bc596ebbe0 ("drm/radeon: disable native
backlight control on pre-r6xx asics (v2)") accidently
broke backlight control on old mac laptops that use the
on-GPU backlight controller.

Signed-off-by: Nathan-J. Hirschauer <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
Cc: [e-mail]

drivers/gpu/drm/radeon/radeon_encoders.c

···
 		    (rdev->pdev->subsystem_vendor == 0x1734) &&
 		    (rdev->pdev->subsystem_device == 0x1107))
 			use_bl = false;
+/* Older PPC macs use on-GPU backlight controller */
+#ifndef CONFIG_PPC_PMAC
 		/* disable native backlight control on older asics */
 		else if (rdev->family < CHIP_R600)
 			use_bl = false;
+#endif
 		else
 			use_bl = true;
 	}