drm/radeon: fix bad DMA from INTERRUPT_CNTL2

drm/radeon: fix bad DMA from INTERRUPT_CNTL2

The INTERRUPT_CNTL2 register expects a valid DMA address, but is
currently set with a GPU MC address.  This can cause problems on
systems that detect the resulting DMA read from an invalid address
(found on a Power8 guest).

Instead, use the DMA address of the dummy page because it will always
be safe.

Fixes: 822d7a552516 ("drm/radeon/kms: Add support for interrupts on r6xx/r7xx chips (v3)")
Fixes: 93bdbdb18306 ("drm/radeon/kms: add support for interrupts on SI")
Fixes: f46d632640cf ("drm/radeon: add support for interrupts on CIK (v5)")
Signed-off-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>