package/libdrm: adjust patch switching to pkg-config for libatomic_ops

In commit fa6c7d16 ("libdrm: fix
libatomic_ops linking"), a patch was added to switch to
PKG_CHECK_MODULES() to detect libatomic_ops instead of
AC_CHECK_HEADER.

However, as explained in
https://autotools.io/pkgconfig/pkg_check_modules.html:

"In contrast with almost all of the original macros, though, the default
 action-if-not-found will end the execution with an error for not having
 found the dependency."

This makes the configure script bail out when libatomic_ops is not
available, which is not what we want in libdrm's configure
script. This commit adjusts the PKG_CHECK_MODULES() call to avoid
failing.

Fixes:

  http://autobuild.buildroot.net/results/cea/cea777dc997f86c1122c8b818d264215a0e77e5a/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>