i965: bounds checks while concatenating sysfs paths

i965: bounds checks while concatenating sysfs paths

This adds some missing return value checks for all uses of snprintf in
brw_performance_query.c. This also switches a use of strncpy + strncat
for snprintf for consistency and to avoid the chance of the strncpy
leaving an unterminated string in the dest buffer if the src is too
long.

This issue with strncpy was picked up by Coverity.

CID: 1402201
Signed-off-by: Robert Bragg <robert@sixbynine.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>