drm/i915: skylake sprite plane scaling using shared scalers

This patch enables skylake sprite plane display scaling using shared
scalers atomic desgin.

v2:
-use single copy of scaler limits (Matt)

v3:
-detaching scalers moved to crtc commit path (Matt)

v4:
-changes to align with updated scaler structures (Matt, me)
-keep sprite src rect in 16.16 format (Matt, Daniel)

v5:
-rebased on top of 90/270 rotation changes (me)
-Refactored skl_update_plane to reduce its size (Daniel)
 It is a step towards having a single function covering all planes.

Signed-off-by: Chandra Konduru <chandra.konduru@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Testcase: igt/kms_plane_scaling
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>