drm/amdgpu: Allow switching to CUSTOM profile on Vega20

Vega20 stores a CUSTOM profile on the GPU, but it may not be valid. Add
a bool to vega20_hwmgr to determine whether or not a valid CUSTOM
profile has been set, and use that to check when a user requests
switching to the CUSTOM profile without passing in any arguments. Then
if the CUSTOM profile has been set already, we can switch to it without
providing the parameters again

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>