pad: gst_pad_activate_mode() always succeed if same mode

pad: gst_pad_activate_mode() always succeed if same mode

Checking that the pad is in the correct mode before the parent is
checked makes the call always succeed if the mode is ok.

This fixes a race with ghostpad where gst_pad_activate_mode() could
trigger a g_critical() if the ghostpad is unparented while the
proxypad is deactivating, for instance if the ghostpad is released.
More specifically, gst_ghost_pad_internal_activate_push_default()'s
call to gst_pad_activate_mode() would fail if ghostpad doesn't have a
parent. With this patch it will return true of mode is already
correct.