amdkfd: Set *buffer_ptr to NULL in case of error

In function acquire_packet_buffer() we may return -ENOMEM. In that case, we
should set the *buffer_ptr to NULL, so that calling functions which check the
*buffer_ptr value as a criteria for success, will know that
acquire_packet_buffer() failed.

Reviewed-by: Alexey Skidanov <alexey.skidanov@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>