drm/amdgpu: fix out of bounds write

Size can be any value and is user controlled resulting in overwriting the
40 byte array wr_buf with an arbitrary length of data from buf.

Signed-off-by: Thelford Williams <tdwilliamsiv@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org