drm/i915: Fix PLL 8x/3 divider for MIPI video mode

MIPI Video Mode for high res panels (requiring dual link), need a
8X/3 divider to be programmed as 0x2. Modifying the same
in this patch.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486551058-22596-3-git-send-email-vidya.srinivas@intel.com