lib/igt_fb: use the entire 32 bit for fnv1a_crc

The function was implemented as a 32-bit hashing function. Commit
85f4c100 ("lib/igt_fb: Ignore the X component when computing CRC")
broke this behavior by hashing 8-bit portions. Restore the 32-bit hasing
behavior while still ignoring the X compoment.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>