drm/amd/display: Use is_dig_enable function instead of dcn10 hardcode

[Why]
This can differ depending on ASIC and we can end up skipping all
transmitter control if we're relying on the wrong bit as a shortcut
for whether link is on/off.

[How]
The ASIC table itself provides the correct DIG check, use that instead
of the dcn10 hardcode.

Reviewed-by: Syed Hassan <syed.hassan@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>