UPSTREAM: coresight: tmc-etr: Refactor function tmc_etr_setup_perf_buf()

Refactoring function tmc_etr_setup_perf_buf() so that it only deals
with the high level etr_perf_buffer, leaving the allocation of the
backend buffer (i.e etr_buf) to another function.

That way the backend buffer allocation function can decide if it wants
to reuse an existing buffer (CPU-wide trace scenarios) or simply create
a new one.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Robert Walker <robert.walker@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(Upstream commit 855ab61c16bf70b646fa9c84e7b07d14a003cd42).

Bug: 140266694
Change-Id: Ib9306a443e0b915a54a918803789441254da4f10
Signed-off-by: Yabin Cui <yabinc@google.com>