drm/atomic-helper: Realign function parameters

Too jarring.

Fixes: f869a6ec ("drm/atomic: Add target_vblank support in atomic helpers (v2)")
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170627145936.18983-14-daniel.vetter@ffwll.ch