There's little point in calling _eglGetNativePlatform() in
eglCopyBuffers. The platform returned should be identical to the one
already stored in our _EGLDisplay.

In the following corner case, the check is incorrect.

The function _eglGetNativePlatform effectively invokes the old-style
eglGetDisplay platform selection. Thus if the EGL_PLATFORM platform does
not match with the EGL_EXT_platform_* used to create the display we'll
error out.

Addresses the egl-copy-buffers piglit test.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>