Add support for reading the CRC in non-blocking mode. Useful for tests
that want to start the CRC capture, then do a bunch of operations, then
collect however many CRCs that got generated. The current
igt_pipe_crc_new() + igt_pipe_crc_get_crcs() method would block until
it gets the requested number of CRCs, whreas in non-blocking mode we
can just read as many as got generated thus far.

v2: __attribute__((warn_unused_result)), document the
    new igt_pipe_crc_get_crcs() return value (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>