drm/i915: Fix frontbuffer false positve.

drm/i915: Fix frontbuffer false positve.

This return 0 without setting atomic bits on fb == crtc->cursor->fb
where causing frontbuffer false positives.

According to Daniel:

The original regression seems to have been introduced in the original
check/commit split:

commit 757f9a3e5b8a812af0c213099a5b31cb423f4d3c
Author: Gustavo Padovan <[email protected]>
Date:   Wed Sep 24 14:20:24 2014 -0300

    drm/i915: move check of intel_crtc_cursor_set_obj() out

Which already cause other trouble, resulting in the check getting moved in

commit e391ea882b1a04fb3f559287ac694652a3cd9da9
Author: Gustavo Padovan <[email protected]>
Date:   Wed Sep 24 14:20:25 2014 -0300

    drm/i915: Fix not checking cursor and object sizes

The frontbuffer tracking itself only was broken when we shifted it into
the check/commit logic with:

commit 32b7eeec4d1e861230b09d437e95d76c86ff4a68
Author: Matt Roper <[email protected]>
Date:   Wed Dec 24 07:59:06 2014 -0800

    drm/i915: Refactor work that can sleep out of commit (v7)

v2: When putting more debug prints I notice the solution was simpler
than I thought. AMS design is solid, just this return was wrong.
Sorry for the noise.

v3: Remove the entire chunck that would probably
    be removed by gcc anyway. (by Daniel)

Cc: Jani Nikula <[email protected]>
Cc: Gustavo Padovan <[email protected]>
Cc: Matt Roper <[email protected]>
Signed-off-by: Rodrigo Vivi <[email protected]>
Reviewed-by: Matt Roper <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>