drm/amd/display: Only clear symclk otg flag for HDMI

[Description]
There is a corner case where the symclk otg flag is cleared
when disabling the phantom pipe for subvp (because the phantom
and main pipe share the same link). This is undesired because
we need the maintain the correct symclk otg flag state for
the main pipe.

For now only clear the flag only for HDMI signal type, since
it's only set for HDMI signal type (phantom is virtual). The
ideal solution is to not clear it if the stream is phantom but
currently there's a bug that doesn't allow us to do this. Once
this issue is fixed the proper fix can be implemented.

Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>