drm/amdgpu: use drm_crtc_vblank_{on,off}()

Replace the legacy drm_vblank_{on,off}() with the new helper functions.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465308482-15104-6-git-send-email-gustavo@padovan.org