drm/amd/display: Null check aconnector in event_property_validate

drm/amd/display: Null check aconnector in event_property_validate

[Why]
previously event_property_validate was only called after we enabled the display.
But after "Refactor HDCP to handle multiple displays per link" this function
can be called at any time. In certain cases we don't have a aconnector

[How]
Null check aconnector and exit early. This is ok because we only need to check the
ENABLED->DESIRED transition if a connector exists.

Fixes: b1abe5586ffc ("drm/amd/display: Refactor HDCP to handle multiple displays per link")
Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>