drm/i915: properly check for pipe count

As suggested by Chris Wilson and Daniel Vetter, this chunk of code can be
simplified with a more simple check.

Also, as noticed by Jesse Barnes, it is worth mentioning that plane is an
enum and num_pipe is an int, so we could be more paranoid here about those
validation checks eventually.

CC: Daniel Vetter <daniel.vetter@ffwll.ch>
CC: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>