drm/i915: Use helper to set CRTC state's mode

We need to call drm_atomic_set_mode_for_crtc() rather than copying the
mode in manually.  As of commit

        commit 99cf4a29
        Author: Daniel Stone <daniels@collabora.com>
        Date:   Mon May 25 19:11:51 2015 +0100

            drm/atomic: Add current-mode blob to CRTC state

the helper now also takes care of setting up the mode property blob for
us; if we don't use the helper and never setup the mode blob, this will
also trigger a failure in drm_atomic_crtc_check() when we have the
DRIVER_ATOMIC flag set (i.e., when using the nuclear pageflip support
via i915.nuclear_pageflip kernel command line parameter).

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>