drm/rockchip: Check for fast link training before enabling psr

Once we start shutting off the link during PSR, we're going to want fast
training to work. If the display doesn't support fast training, don't
enable psr.

Changes in v2:
- None
Changes in v3:
- None
Changes in v4:
- None
Changes in v5:
- None

Link to v1: https://patchwork.freedesktop.org/patch/msgid/20190228210939.83386-3-sean@poorly.run
Link to v2: https://patchwork.freedesktop.org/patch/msgid/20190326204509.96515-2-sean@poorly.run
Link to v3: https://patchwork.freedesktop.org/patch/msgid/20190502194956.218441-9-sean@poorly.run
Link to v4: https://patchwork.freedesktop.org/patch/msgid/20190508160920.144739-8-sean@poorly.run

Cc: Zain Wang <wzz@rock-chips.com>
Cc: Tomasz Figa <tfiga@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611160844.257498-8-sean@poorly.run