In an upcoming patch we'll move this calculation to the atomic 'check'
phase so that the display update can be rejected early if no valid
watermark programming is possible.

v2:
 - Drop intel_pstate_for_cstate_plane() helper and add note about how
   the code needs to evolve in the future if we start allowing more than
   one pending commit against a CRTC.  (Maarten)

v3:
 - Only have skl_compute_wm_level calculate watermarks for enabled
   planes; we can just set the other planes on a CRTC to disabled
   without having to look at the plane state.  This is important because
   despite our CRTC lock we can still have racing commits that modify
   a disabled plane's property without turning it on.  (Maarten)
Signed-off-by: NMatt Roper <matthew.d.roper@intel.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-13-git-send-email-matthew.d.roper@intel.com