drm/i915: Fix cursor visibility checks also for the right/bottom screen edges

First of all we should not be looking at fb->{width,height} as those do
not tell us what the actual pipe size is. Second of all we need to use
>= for the comparison.

So fix the comparison, and make use of the new pipe_src_{w,h} to
determine the real pipe source dimensions.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>