drm/amd/display: add dmcub check on RENOIR

drm/amd/display: add dmcub check on RENOIR

[ Upstream commit 3b2e973dff59d88bee1d814ddf8762a24fc02b60 ]

RENOIR loads dmub fw not dmcu, check dmcu only will prevent loading iram,
it breaks backlight control.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=208277
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>