drm/amd/display: Add get_dig_frontend implementation for DCEx

Some old ASICs might not implement/require get_dig_frontend helper; in
this scenario, we can have a NULL pointer exception when we try to call
it inside vbios disable operation. For example, this situation might
happen when using Polaris12 with an eDP panel. This commit avoids this
situation by adding a specific get_dig_frontend implementation for DCEx.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Harry Wentland <Harry.Wentland@amd.com>
Cc: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Cc: Chiawen Huang <chiawen.huang@amd.com>
Reported-and-tested-by: Borislav Petkov <bp@suse.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org