Because of hash collisions tests should only ever compare crc
checksums for equality. Checking for inequality can result in random
failures.

To ensure this only expose and igt_assert function and use that.
Follow-up patches will rework the code for tests which don't follow
this requirement and try to compare for CRC inequality.

v2: Rebase on top of Matt's kms_plane changes.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>