drm/i915: Clear crtc atomic flags at beginning of transaction

Once we have full atomic modeset, these kind of flags should be in a
real intel_crtc_state that's tracked properly.  In the meantime, make
sure we clear out any old flags at the beginning of a transaction so
that we don't wind up seeing leftover flags from old transactions that
were checked, but never went to the commit step.  At the moment, a
failed check or prepare could leave stale flags behind that interfere
with the next atomic transaction.

v2: Just do a memset; the series this patch was originally part of
    placed additional fields into the structure that shouldn't be
    cleared, but that's no longer the case.
Signed-off-by: NMatt Roper <matthew.d.roper@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_atomic.c

@@ -76,6 +76,8 @@ int intel_atomic_check(struct drm_device *dev,
 	state->allow_modeset = false;
 	for (i = 0; i < ncrtcs; i++) {
 		struct intel_crtc *crtc = to_intel_crtc(state->crtcs[i]);
+		if (crtc)
+			memset(&crtc->atomic, 0, sizeof(crtc->atomic));
 		if (crtc && crtc->pipe != nuclear_pipe)
 			not_nuclear = true;
 	}