drm/i915/dp: cache source rates at init

We need the source rates array so often that it makes sense to set it
once at init. This reduces function calls when we need the rates, making
the code easier to follow.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/aa998882d2b824f671272c60e9d26621ab9d2d17.1490712890.git.jani.nikula@intel.com