drm/i915: Fix scaler init during CRTC HW state readout

drm/i915: Fix scaler init during CRTC HW state readout

The scaler allocation code depends on a non-zero default value for the
crtc scaler_id, so make sure we initialize the scaler state accordingly
even if the crtc is off. This fixes at least an initial YUV420 modeset
(added in a follow-up patchset by Shashank) when booting with the screen
off: after the initial HW readout and modeset which enables the scaler a
subsequent modeset will disable the scaler which isn't properly
allocated. This results in a funky HW state where the pipe scaler HW
registers can't be modified and the normally black screen is grey and
shifted to the right or jitters.

The problem was revealed by Shashank's YUV420 patchset and first
reported by Ville.

v2:
- In the stable tag also include versions which need backporting (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chandra Konduru <chandra.konduru@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: <stable@vger.kernel.org> # 4.2.x
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: a1b2278e4dfc ("drm/i915: skylake panel fitting using shared scalers")
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720112820.26816-1-imre.deak@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>