drm/radeon: implement pci config reset for r6xx/7xx (v3)

pci config reset is a low level reset that resets
the entire chip from the bus interface.  It can
be more reliable if soft reset fails.

There's not much information still available on
r6xx, so r6xx is based on guess-work.

v2: put behind module parameter
v3: add IGP check

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>