drm/i915: setup scalers for crtc_compute_config

Added intel_atomic_setup_scalers to setup scalers based on
staged scaling requests from a crtc and its planes. If staged
requests are supportable, this function assigns scalers to
requested planes and crtc. Note that the scaler assignement
itself is staged into crtc_state and respective plane_states
for later commit after all checks have been done.

overall high level flow:
 - scaler requests are staged into crtc_state by planes/crtc
 - check whether staged scaling requests can be supported
 - add planes using scalers that aren't in current transaction
 - assign scalers to requested users
 - as part of plane commit, scalers will be committed
   (i.e., either attached or detached) to respective planes in hw
 - as part of crtc_commit, scaler will be either attached or detached
   to crtc in hw

crtc_compute_config calls intel_atomic_setup_scalers() to start
scaler assignments as per scaler state in crtc config. This call
should be moved to atomic crtc once it is available.

v2:
-removed a log message (me)
-changed input parameter to crtc_state (me)

v3:
-remove assigning plane_state returned by drm_atomic_get_plane_state (Matt)
-fail if there is an error from drm_atomic_get_plane_state (Matt)

v4:
-changes to align with updated scaler structure (Matt, me)

v5:
-added addtional checks before enabling HQ mode (me)
-added comments to enable HQ mode (Matt)

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>