drm/radeon: make missing smc ucode non-fatal (r7xx-SI)

Prevent driver load problems if the smc is missing.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=63011



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Cc: stable@vger.kernel.org