drm/fb_helper: Use correct allocation count for arrays.

The fb helper iterates over connectors, using fb_helper->num_connectors
makes it more clear what size the allocations are.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455540316-17066-1-git-send-email-maarten.lankhorst@linux.intel.com