drm/i915/glk: Add DSI PLL divider range for glk

drm/i915/glk: Add DSI PLL divider range for glk

PLL divider range for GLK is different than that of
BXT, hence adding the GLK range check in this patch.

v2: Code restructure using min and max ratio variables (Ander)
v3: Code changes to avoid "maybe-uninitialized" warning (Jani)

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/1487335415-14766-5-git-send-email-madhav.chauhan@intel.com