drm/kms/radeon: Reorder vblank and pageflip interrupt handling.

In the vblank irq handler, calls to actual vblank handling,
or at least drm_handle_vblank(), need to happen before
calls to radeon_crtc_handle_flip().

Reason: The high precision pageflip timestamping
and some other pageflip optimizations will need the updated
vblank count and timestamps for the current vblank interval.

These are calculated in drm_handle_vblank(), therefore it
must go first.

Signed-off-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>