drm/i915: Refactor timestamping constants update

Once we do the hw vs. uapi split we can no longer use
drm_atomic_helper_calc_timestamping_constants() as it'll
consult the uapi state instead of the hw state.

So let's just update the vblank timestamping constants whenever
we update the scanline offset. We use both to convert the hw
scanline count to something which matches the software timing
values.

First I thought to put these into intel_crtc_vblank_on() but
we may want to get the scanline counter value before that (eg.
from some early tracepoints), so let's stick to updating them
a bit earlier than intel_crtc_vblank_on().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191007114943.29307-3-ville.syrjala@linux.intel.com