drm/amd/display: Fallback to DMCUB when command table is missing

[Why]
If the command table isn't available then we can fallback to DMCUB
offloading if it's enabled and available.

[How]
Instead of assigning NULL for supported command table functions we can
fallback to the DMCUB when it's available.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>