drm/i915: make IVB FDI training match spec v3

drm/i915: make IVB FDI training match spec v3

The existing code was trying different vswing and preemphasis settings
in the wrong place, and wasn't trying them enough.  So add a loop to
walk through them, properly disabling FDI TX and RX in between if a
failure is detected.

v2: remove unneeded reg writes, add delays around bit lock checks (Jesse)
v3: fix TX and RX disable per spec (Paulo)
    fix delays per spec (Paulo)
    make RX symbol lock check match TX bit lock check (Paulo)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51983
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>