drm/i915i/glk: Program MIPI_CLOCK_CTRL only for BXT

Register MIPI_CLOCK_CTRL is applicable only
for BXT platform. Future platform have other
registers to program the escape clock dividers.

Signed-off-by: Deepak M <m.deepak@intel.com>
Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487335415-14766-6-git-send-email-madhav.chauhan@intel.com