msm: npu: Allocate buffer per client for register read

In order to avoid double free or use after free the buffer
allocated while reading registers, it is needed to allocate the
buffer per client and free it in release function.

Change-Id: I96b6b8671b517f9123b28ce07f546de06fe19eab
Signed-off-by: Da Hoon Pyun <dpyun@codeaurora.org>