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: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>