drm/i915: Call g4x_find_best_dpll() directly from ILK+ code

The call to dev_priv->display.find_dpll() is already in platform
specific code, so avoid the extra detour.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-5-git-send-email-ander.conselvan.de.oliveira@intel.com