drm/amdgpu: Allow switching to CUSTOM profile on Vega10 v2

Don't return an error if the CUSTOM profile is selected, just apply it
with the values saved to the GPU. But ensure that we zero out the
copy stored in adev to ensure that a valid profile has been submitted at
some point first

v2: Fix comment that wasn't updated from previous patch

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>