drm: i915: reduce memory footprint when 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>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Add cast since %*ph expects and int for the size parameter.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>