drm/radeon/kms: don't try to be smart in the hpd handler

Attempting to try and turn off disconnected display hw in the
hotput handler lead to more problems than it helped.  For
now just register an event and only attempt the do something
interesting with DP.  Other connectors are just too problematic:
- Some systems have an HPD pin assigned to LVDS, but it's rarely
if ever connected properly and we don't really care about hpd
events on LVDS anyway since it's always connected.
- The HPD pin is wired up correctly for eDP, but we don't really
have to do anything since the events since it's always connected.
- Some HPD pins fire more than once when you connect/disconnect
- etc.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=39882

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

drivers/gpu/drm/radeon/radeon_mode.h

@@ -479,6 +479,7 @@ extern void radeon_dp_set_link_config(struct drm_connector *connector,
 				      struct drm_display_mode *mode);
 extern void radeon_dp_link_train(struct drm_encoder *encoder,
 				 struct drm_connector *connector);
+extern bool radeon_dp_needs_link_train(struct radeon_connector *radeon_connector);
 extern u8 radeon_dp_getsinktype(struct radeon_connector *radeon_connector);
 extern bool radeon_dp_getdpcd(struct radeon_connector *radeon_connector);
 extern void atombios_dig_encoder_setup(struct drm_encoder *encoder, int action, int panel_mode);