drm/i915/glk: Configure number of sprite planes properly

Geminilake has 4 planes (3 sprites) per pipe.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480667037-11215-10-git-send-email-ander.conselvan.de.oliveira@intel.com