drm/i915/glk: Validate only DSI PORT A PLL divider

As per BSPEC, GLK supports MIPI DSI 8X clk only on PORT A.
Therefore only for PORT A PLL divider value should be validated.

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/1487335415-14766-8-git-send-email-madhav.chauhan@intel.com