drm/amd/display: Don't ask PSP to load DMCUB for backdoor load

[Why]
If we're doing backdoor load then do it entirely ourselves without
invoking any of the frontdoor path to avoid potential issues with
outdated tOS.

[How]
Check the load type and don't pass it to base if we don't want it
loaded.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>