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

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

[ Upstream commit ad309284a52be47c8b3126c9376358bf381861bc ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>