drm/radeon/atom: workaround vbios bug in transmitter table on rs780

Some rs780 asics seem to be affected as well.

See:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=91f3a6aaf280294b07c05dfe606e6c27b7ba3c72

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=60791



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org