drm/amd/powerplay: report bootup clock as max supported on dpm disabled

With gfxclk or uclk dpm disabled, it's reasonable to report bootup clock
as the max supported.

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