egl/dri2: Dispatch eglCreatePixmapSurface by display, not driver

Add dri2_egl_display_vtbl::create_pbuffer_surface, set it for each
platform, and let egl_dri2 dispatch eglCreatePixmapSurface to that.

This prepares for the EGL platform extensions.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
7 files changed