libhwc2.1: avoid finding std::map element twice

We should not use std::map::at() after calling std::map::find() for the
same key. Otherwise, it would find the same element twice and it's
unnecessary.

Bug: 295892886
Test: trigger assignResource using hwc-tester & check simpleperf
Change-Id: I971965a8b2a84120c989e55b87e223225ea32456
1 file changed