drm/i915/glk: Calculate high/low switch count for GLK

As per BSPEC, high/low switch count to be programmed in
terms of byteclock using exit_zero_count and prep_count.
For Geminilake exit/prep counts are already calculated
in terms of byteclock. This patch calculates high/low
switch count using counts value in byteclock, old calculation
leads to screen flicker/shift issue while resuming from S3/S4.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494336565-19185-1-git-send-email-madhav.chauhan@intel.com