drm/radeon/kms: bail on BTC parts if MC ucode is missing

commit 77e00f2e upstream.

We already do this for cayman, need to also do it for
BTC parts.  The default memory and voltage setup is not
adequate for advanced operation.  Continuing will
result in an unusable display.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>