drm/i915/fbc: Remove second redundant intel_fbc_pre_update() call

drm/i915/fbc: Remove second redundant intel_fbc_pre_update() call

I fumbled the conflict resolution a bit when applying the
fbc vblank wait w/a. Because of that we now call intel_fbc_pre_update()
twice. Remove the second redundant call.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213133453.22152-2-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>