drm/i915/gen9+: Use cstate plane mask instead of crtc->state.

I'm planning on getting rid of all obj->state dereferences,
and replace thhem with accessor functions.
Remove this one early, they're equivalent because removed
planes are already part of the state, else they could not
have been removed.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477489299-25777-3-git-send-email-maarten.lankhorst@linux.intel.com


Reviewed-by: Matt Roper <matthew.d.roper@intel.com>