drm/i915: Remove unused 'in_vbl' from i915_get_crtc_scanoutpos()

Commit 1bf6ad62 ("drm/vblank: drop the mode argument from
drm_calc_vbltimestamp_from_scanoutpos") removed the use of in_vbl, but
did not remove the local variable. Do so now.

Fixes: 1bf6ad62

 ("drm/vblank: drop the mode argument from drm_calc_vbltimestamp_from_scanoutpos")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170914164213.18461-1-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>