drm/amd/display: check TG is non-null before checking if enabled

stable inclusion
from stable-v5.10.193
commit 61c303cfb1353754831aaa82ead4e698d2f6f5ae
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I9399M

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=61c303cfb1353754831aaa82ead4e698d2f6f5ae



--------------------------------

[ Upstream commit 5a25cefc ]

[Why & How]
If there is no TG allocation we can dereference a NULL pointer when
checking if the TG is enabled.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Taimur Hassan <syed.hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>