drm: properly init/reset connector status

This can help drivers to make somewhat intelligent decisions in their
->detect callback: If the connector is hpd capable and in the unknown
state, the driver needs to force a full detect cycle. Otherwise it
could just (if it chooses so) to update the connector state from it's
hpd handler directly, and always return that in the ->detect callback.

Atm only drm/i915 calls drm_mode_config_reset at resume time, so other
drivers would need to add that call first before using this facility.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>