drm/i915: Use num_scalers instead of SKL_NUM_SCALERS in debugfs

Better to use num_scaler itself while printing scaler_info.
This fixes a bug of printing information for the missing
second scaler on pipe C for SKL platform.

Signed-off-by: A.Sunil Kamath <sunil.kamath@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479664226-22307-1-git-send-email-sunil.kamath@intel.com