gl-renderer: do not expose query_dmabuf_formats and query_dmabuf_modifiers

In commit "libweston: add struct weston_drm_format" struct
weston_drm_format and its helper functions were added to libweston.

The functions query_dmabuf_formats and query_dmabuf_modifiers are very
specific to GL-renderer and its internals. So instead of exposing them
in libweston, query and store DRM formats and modifiers internally in
GL-renderer. Also, add a vfunction to struct weston_renderer in order
to retrieve the formats.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>