egl: return correct error for EGL_KHR_image_pixmap

egl: return correct error for EGL_KHR_image_pixmap

EGL_KHR_image_pixmap says:
> If <target> is EGL_NATIVE_PIXMAP_KHR, and <ctx> is not EGL_NO_CONTEXT,
> the error EGL_BAD_PARAMETER is generated.

Reported-by: Rohan Garg <rohan@garg.io>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/166
Fixes: a1c4a8a3c855d52fbfef "egl: Add support for EGL_KHR_image."
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/454>