drm/i915/dp: cache common rates with sink rates

Now that source rates are static and sink rates are updated whenever
DPCD is updated, we can do and cache the intersection of them whenever
sink rates are updated. This reduces code complexity, as we don't have
to keep calling the functions to intersect. We also get rid of several
common rates arrays on stack.

Limiting the common rates by a max link rate can be done by picking the
first N elements of the cached common rates.

v2: get rid of the local common_rates variable (Manasi)
v3: don't clobber cached eDP rates on short pulse (Ville)

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/e3b287e8cb6559b1f8fd4e80b78a8d22f1802eb7.1491485983.git.jani.nikula@intel.com