drm/amd/display: add hdmi disable debug check

[Why]
Using the hdmi_disable option doesnt disable 6GB bandwidth

[How]
Add debug.hdmi20_disable flage when checking 6GB enable or not.

Reviewed-by: Chris Park <Chris.Park@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Dale Zhao <dale.zhao@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>