drm/i915/dp: abstract rate array length limiting

This will be useful later on. Also move the functions around to not need
forward declarations in subsequent patches. No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/40f37f08cad33234cd86337d39e823ac6e55805f.1517482774.git.jani.nikula@intel.com