drm/radeon: make missing smc ucode non-fatal

The smc ucode is required for dpm (dynamic power
management), but if it's missing just skip dpm setup
and don't disable acceleration.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=67876



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>