drm/radeon: call hpd_irq_event on resume

commit dbb17a21c131eca94eb31136eee9a7fe5aff00d9 upstream.

Need to call this on resume if displays changes during
suspend in order to properly be notified of changes.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc drivers/gpu/drm/radeon/radeon_device.c
Simple merge