drm/i915: skylake panel fitting using shared scalers

Enabling skylake panel fitting feature using shared scalers

v2:
-added force detach parameter for pfit disable purpose (me)
-read crtc scaler state from hw state (Daniel)
-replaced both skylake_pfit_enable and disable with skylake_pfit_update (me)
-added scaler id check to intel_pipe_config_compare (Daniel)

v3:
-updated function header to kerneldoc format (Matt)
-dropped need_scaling checks (Matt)

v4:
-move clearing of scaler id from commit path to check path (Matt)
-updated colorkey checks based on recent updates (me)
-squashed scaler check while enabling colorkey to here (me)
-use values in plane_state->src as regular integers (me)
-changes made not to modify state in commit path (Matt)

v5:
-squashed helper function to update scaler users to here (Matt)
-squashed helper function to detach scaler to here (Matt, me)
-changes to align with updated scaler structures (Matt, me)

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