drm/i915: Fix skl+ non-scaled pfit modes

Fix skl_update_scaler_crtc() to deal with different scaling
modes correctly. The current implementation assumes
DRM_MODE_SCALE_FULLSCREEN. Fortunately we don't expose any
border properties currently so the code does actually end
up doing the right thing (assigning a scaler for pfit).
The code does need to be fixed before any borders are
exposed.

Also we have redundant calls to skl_update_scaler_crtc() in
dp/hdmi .compute_config() which can be nuked. They were anyway
called before we had even computed the pfit state so were
basically nonsense. The real call we need to keep is in
intel_crtc_atomic_check().

v2: Deal witrh skl_update_scaler_crtc() in intel_dp_ycbcr420_config()

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200422161917.17389-1-ville.syrjala@linux.intel.com