drm/i915/selftests: fix uninitialized variable sum when summing up values

Currently the variable sum is not uninitialized and hence will cause an
incorrect result in the summation values.  Fix this by initializing
sum to the first item in the summation.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 3c7a44bb ("drm/i915/selftests: Perform some basic cycle counting of MI ops")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210143205.338308-1-colin.king@canonical.com