drm/i915: Read PSR caps/intermediate freqs/etc. only once on eDP

Currently we re-read a bunch of static eDP panel caps from the DPCD
over and over again. Let's do it only once to save some time and effort.

v2: Make thing less confusing with intel_edp_init_dpcd() (Chris)
    Move no_aux_handshake setup in there as well
v3: Move tps3/rate printout to intel_dp_long_pulse() so that
    we'll still get them on eDP as well

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1469800359-7087-1-git-send-email-ville.syrjala@linux.intel.com