glamor/glamor_egl.c: EGL_NATIVE_PIXMAP_KHR do not require contexts

From https://www.khronos.org/registry/EGL/extensions/KHR/EGL_KHR_image_pixmap.txt

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

Fixes: a5321ea4 ("Allow to create textured pixmaps from gbm_bo without using gem names")