drm/radeon/kms: read back register before writing in IIO.

This fixes RH bugzilla #527874.

On resume the atom posting wasn't working, however vbe posting was
going fine, after 2 weeks over irc, and 8 hrs with the hardware,
I tracked it down to the memory device table and it access the MC
registers via IIO, it appears the rv515 atom iio table might not
be fully functional, so adding a readback before doing a write
either provides enough delay to make things resume correctly.

Thanks to Peng Huang at Red Hat for coming to Brisbane.

Signed-off-by: Dave Airlie <airlied@redhat.com>