drm/i915: Kill intel_crtc->cursor_bo

The vma may have been rebound between the last time the cursor was
enabled and now, so skipping the cursor gtt offset deduction is not
safe unless we would also reset cursor_bo to NULL when disabling the
cursor. Just thow cursor_bo to the bin instead since it's lost all
other uses thanks to universal plane support.

Chris pointed out that cursor updates are currently too slow
via universal planes that micro optimizations like these wouldn't
even help.

v2: Add a note about futility of micro optimizations (Chris)

Cc: drm-intel-fixes@lists.freedesktop.org
References: http://lists.freedesktop.org/archives/intel-gfx/2015-December/082976.html
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450107302-17171-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>