drm/i915/dp: Fix the order of platforms for setting DP source rates

The usual if ladder order should be from newest to oldest
platform. However the CNL conditional statement was misplaced.
This patch sets the DP source for platforms starting from the newest
to oldest.

Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1519701075-9894-1-git-send-email-manasi.d.navare@intel.com