drm/radeon/kms/igp: fix possible divide by 0 in bandwidth code (v2)

Some IGP systems specify the system memory clock in the Firmware
table rather than the IGP info table.  Check both and make sure
we have a value system memory clock value.

v2: make sure rs690_pm_info is called on rs780/rs880 as well.

fixes a regression since 07d4190327b02ab3aaad25a2d168f79d92e8f8c2.

Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Tested-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>