drm/amd/display: Enable use of dmub iff dmcu is disabled

[Why & How]
DMUB command table should be allowed to be used
only if dmcu is explicitly disabled.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>