drm/i915: Plumb crtc_state to link training

Get rid of mode crtc->config usage, and some ad-hoc intel_dp state
usage by plumbing the crtc state all the way down to the link training
code.

Unfortunately we do have to keep some cached state in intel_dp so
that we can do the "does the link need retraining?" checks from
the short hpd handler.

v2: Add intel_crtc_state forward declaration
v3: Don't kill the PHY test code totally since it's
    now in the hotplug work where we can get at the states
v4: Don't resurrect the debug scrambling disable bit (Imre)
    Use intel_dp_mst_is_master_trans() (Imre)

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201001111053.24451-1-ville.syrjala@linux.intel.com