drm/radeon: Fix spurious unplug event on radeon HDMI

On several HP models (tested on HP 3125 and HP Probook 455 G2),
spurious unplug events are emitted upon login on Chrome OS.
This is likely due to the way Chrome OS restarts graphics
upon login, so it's possible it's an issue on other
distributions but not as common, though I haven't
reproduced the issue elsewhere.
Use logic from an earlier version of the merged change
(see link below) which iterates over connectors and finds
matching encoders, rather than the other way around.
Also fixes an issue with screen mirroring on Chrome OS.
I've deployed this patch on Fedora and did not observe
any regression on these devices.

Link: drm/amd#1569 (comment 1603002)
Link: drm/amd#3771


Fixes: 20ea3471 ("drm/radeon: Add HD-audio component notifier support (v6)")
Signed-off-by: Steven 'Steve' Kendall <skend@chromium.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>