intel/perf: fix EuThreadsCount value in performance equations

EuThreadsCount is supposed to be the number of threads per EU, not the
total number of threads in the whole device.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 1fc7b951 ("i965: Add Gen8+ INTEL_performance_query support")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 0430c6d1)