drm/i915: Calculate vblank timestamping constants before enabling vblank.

This is required to properly initialize vblanks on the active crtc.
Without it drm_calc_vbltimestamp_from_scanoutpos can fail with
crtc 0: Noop due to uninitialized mode.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>