drm/radeon/kms: add ioport register access

This is required for the NB_MISC regs on rs780/rs880 which
means HDMI/DVI/DP ports using PCIEPHY won't work without
it. It might also help with s/r (asic init) issues on other
atombios cards.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=28774
and similar issues reported by Alberto Milone.

[airlied: Squash io fix patch]

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Tested-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>