drm/radeon: handle broken disabled rb mask gracefully (6xx/7xx) (v2)

This is a port of cedb655a
to older asics.  Fixes a possible divide by 0 if the harvest
register is invalid.

v2: drop some additional harvest munging.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org