drm/amd: add some extra checks that is_dig_enabled is defined

There are a few places that this isn't checked that could potentially
be a NULL pointer access.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>