drm/i915/psr: Try to program link training times correctly

commit 03b7b5f9 upstream.

The default of 0 is 500us of link training, but that's not enough for
some platforms. Decoding this correctly means we're using 2.5ms of
link training on these platforms, which fixes flickering issues
associated with enabling PSR.

v2: Unbotch the math a bit.

v3: Drop debug hunk.

v4: Improve commit message.

Tested-by: Lyude <cpaul@redhat.com>
Cc: Lyude <cpaul@redhat.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95176


Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Sonika Jindal <sonika.jindal@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Cc: "Pandiyan, Dhinakaran" <dhinakaran.pandiyan@intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by:  <fritsch@kodi.tv>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463590036-17824-2-git-send-email-daniel.vetter@ffwll.ch


(cherry picked from commit 50db1390)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>