drivers: qcom: rpmh: Avoid allocating memory on heap for each request

Allocate payload data statically as part of the structure over
allocating memory on the heap for each request and releasing them. This
should help avoid fragmenting the kernel memory and speed up the
RPMH request operation a bit while simplifying the code, a bit.

Change-Id: Ia0baf8c1313d017e6d43ff6e4f7ee9cb8e71c9ce
Signed-off-by: Lina Iyer <ilina@codeaurora.org>