drm/i915: Fix vblank timestamps with VRR

To get sensible vblank timestamping behaviour we need to feed
the vmax based timings to the vblank code, otherwise it'll chop
off the scanline counter when it exceeds the minumum vtotal.

Additionally with VRR we have three cases to consider when we
generate the vblank timestamp:
1) we are in vertical active
  -> nothing special needs to be done, just return the current
     scanout position and the core will calculate the timestamp
     corresponding to the past time when the current vertical
     active started
2) we are in vertical blank and no push has been sent
  -> the hardware will keep extending the vblank presumably
     to its maximum length, so we make the timestmap match the
     expected time when the max length vblank will end. Since
     the timings used for this are now based on vmax nothing
     special actually needs to be done
3) we are in vblank and a push has been sent so the vblank is
   about to terminate
  -> presumably we want the timestmap to accurately reflect
     when the vblank will terminate, so we use the sampled
     frame timestamp vs. current timestamp to guesstimate
     how far along the vblank exit we are, and then we
     adjust the reported scanout position accordingly so
     that the core will see that the vblank is close to
     ending.

v2:
* Fix the else if (use_scanline_Counter) (Manasi)

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122232647.22688-17-manasi.d.navare@intel.com