drm/i915/glk: Allow dotclock up to 2 * cdclk on geminilake

Geminilake has double wide pipes so it can output two pixels per CD
clock.

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/1480667037-11215-8-git-send-email-ander.conselvan.de.oliveira@intel.com