drm/radeon/kms: minor driver cleanups

- Make the logic in r100_pll_errata_after_index() match the other
errata functions
- Use rdev->family rather than rdev->flags & RADEON_FAMILY_MASK for kms
- replace rn50 check using ids with ASIC_IS_RN50 convenience macro

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