perf/test_d007: Update logic for intermediate level

Test 007 iterates over the performance levels discovered to find min,
max and intermediate. The intermediate level is chosen to be the second
one found. In domains where there are only 2 levels the intermediate
level is in fact the max.
In further tests, the intermediate level is chosen to replace the max,
while effectively the value used is the max, thus nullifying the test
case.

This patch adds an extra check to pick the minimum level as
intermediate when a performance domain exposes only two levels.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: If952eaf6a64168c00a7ba7ef75c5a3adf6aee0ef