radeon: introduce a dp_work handler

This is to be called on short HPD irqs, just introduce
the basic infrastructure for it.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/radeon.h
Simple merge

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