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

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

[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>