Generalize finding the rocm_agent_enumberator program

On Fedora, rocminfo is a fedora package and rocm_agent_enumberator is
installed to /usr/bin.  This causes this error when building.

CMake Error at external/llvm-project/mlir/lib/ExecutionEngine/CMakeLists.txt:232 (message):
  Could not run rocm_agent_enumerator and ROCM_TEST_CHIPSET is not defined

So use find_program() to look for rocm_agent_enumerator instead of assuming a
single location.

Signed-off-by: Tom Rix <trix@redhat.com>

Reviewed By: krzysz00

Differential Revision: https://reviews.llvm.org/D156826