drm/i915: Move scanline_offset and timestamping constant setup to intel_modeset_readout_hw_state()

intel_modeset_readout_hw_state() seems like the more appropriate place
for populating the scanline_offset and timestamping constants than
intel_sanitize_crtc() since they are basically part of the state we
read out.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>