drm/radeon: don't set hpd, afmt interrupts when interrupts are disabled

Avoids splatter if the interrupt handler is not registered due
to acceleration being disabled.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable@vger.kernel.org