drm/radeon: Cleanup pageflipping IRQ handling for evergreen, si

Same as the previous patch, but for pageflipping now. This also lets us
clear up the copy paste for vblank/vline IRQs.

Changes since v1:
- Preserve the order all registers are written back

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lyude <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>