drm/radeon: reduce memory footprint for debugging

There is no need to use hex_dump_to_buffer() since we have a kernel helper to
dump up to 64 bytes just via printk(). In our case the actual size is 15 bytes.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>