cmake: Use FindPNG instead of find_path/find_library

cmake: Use FindPNG instead of find_path/find_library

dEQP's use of find_path() and find_library() failed to find the png
paths if the paths contained a version suffix. For example, they failed
to find /usr/include/libpng12/png.h and /usr/lib64/libpng12.so.

CMake ships a standard FindPNG module that does not get confounded by
version-suffixed paths.  The module also respects cross-compilation
environments.

Change-Id: I636be9430697a9d883040b53e79767466fbd2ad5
Signed-off-by: Chad Versace <chad.versace@intel.com>