drm/amd/display: Check for tg ops in dce110_set_avmute

drm/amd/display: Check for tg ops in dce110_set_avmute

Some older DCE timing generators do not implement is_tg_enabled in
their ops table. Calling it unconditionally when waiting for AV mute
frames causes a NULL pointer dereference on Southern Islands dGPUs
when turning the display off over HDMI.

Check that tg and the required ops exist before waiting for frames.

Fixes: 414da24137ac ("drm/amd/display: Add AV mute wait frames to dce110_set_avmute")
Closes: https://gitlab.freedesktop.org/drm/amd/-/work_items/5557
Tested-by: Viktor Jägersküpper <viktor_jaegerskuepper@freenet.de>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 2686a0c0aaa07bec2e24131835cf27b5fd4935a5)
Cc: stable@vger.kernel.org