drm/i915: Fix pixel clock and crtc clock config mismatch

In case of dual link mode, the mode clock that we get
from the VBT is halved.

v2: Simplify the calculation (Jani).

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1556809862-31203-4-git-send-email-vandita.kulkarni@intel.com