drm/vc4: Fix memory leak during gpu reset.

__drm_atomic_helper_crtc_destroy_state does not free memory, it only
cleans it up. Fix this by calling the functions own destroy function.

Fixes: 6d6e5003

 ("drm/vc4: Allocate the right amount of space for boot-time CRTC state.")
Cc: Eric Anholt <eric@anholt.net>
Cc: <stable@vger.kernel.org> # v4.6+
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-2-maarten.lankhorst@linux.intel.com