drm/i915/gen9: Allow watermark calculation on in-flight atomic state (v3)

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: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-13-git-send-email-matthew.d.roper@intel.com