mesa: double check for eglplatform.h

Even if 'egl' is in PACKAGECONFIG, mesa egl support
can be disabled explicitly (changing configure flags
using a .bbappend, for example).
On dora, meta-fsl-arm is an example of this kind.
On master there are no known cases, and we should
encourge package configuration through PACKAGECONFIG.

This patch adds another check for the existence
of eglplatform.h before 'sed' can alter it.

Signed-off-by: Valentin Popa <valentin.popa@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>