drm/i915: Improve the accuracy of get_scanout_pos on CTG+

The DSL register increments at the start of horizontal sync, so it
manages to miss the entire active portion of the current line.

Improve the get_scanoutpos accuracy a bit when the scanout position is
close to the start or end of vblank. We can do that by double checking
the DSL value against the vblank status bit from ISR.

Cc: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by:  <mario.kleiner.de@gmail.com>
Tested-by:  <mario.kleiner.de@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>