drm/amd|radeon: Drop drm_vblank_cleanup

Both drivers shut down all crtc beforehand already, which will shut up
any pending vblank (the only thing vblank_cleanup really does is
disable the disable timer). Hence we don't need this here and can
remove it.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-2-daniel.vetter@ffwll.ch