tests/intel/xe_oa: Keep OA user buffer size dynamic

non-zero-reason OA test has a hardcoded user buffer size for
any kernel OA buffer. This makes the test act differently for
different sizes. Thus make the user buffer size to be in line
with the kernel OA buffer being allocated.

v2: Address operator precedence (Ashutosh)

Signed-off-by: Sai Teja Pottumuttu <sai.teja.pottumuttu@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>