drm/amd/display: Don't lock connection_mutex for DMUB HPD

[Why]
Per DRM spec we only need to hold that lock when touching
connector->state - which we do not do in that handler.

Taking this locking introduces unnecessary dependencies with other
threads which is bad for performance and opens up the potential for
a deadlock since there are multiple locks being held at once.

[How]
Remove the connection_mutex lock/unlock routine and just iterate over
the drm connectors normally. The iter helpers implicitly lock the
connection list so this is safe to do.

DC link access also does not need to be guarded since the link
table is static at creation - we don't dynamically add or remove links,
just streams.

Fixes: e27c41d5 ("drm/amd/display: Support for DMUB HPD interrupt handling")

Reviewed-by: Jude Shih <shenshih@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>