drm/radeon: reintroduce radeon_dp_work_func content

drm/radeon: reintroduce radeon_dp_work_func content

commit a34fc1bcd2c4d8b09dcfc0b95ac65bca1e579bd7 upstream.

Put back the radeon_dp_work_func logic.  It seems that
handling DP RX interrupts is necessary to make some
panels work.  This was removed with the MST support,
but it regresses some systems so add it back.  While
we are here, add the proper mutex locking.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2567
Fixes: 01ad1d9c2888 ("drm/radeon: Drop legacy MST support")
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>