Reduce duplication in graphics module handling

The following discussion from !101 (merged) should be addressed:

• @smcv started a discussion:

  I think we're getting to the point where it would make sense to have an extra member SRT_NUM_GRAPHICS_MODULES at the end of the enum, and an array in this struct:

  typedef struct
  {
    GList *cached;
    gboolean available;
  } CachedModules;
  
  typedef struct
  {
    CachedModules graphics_modules[SRT_NUM_GRAPHICS_MODULES];
  } Abi;

  and then when we're dealing with all the graphics modules as a group (init, finalize, forget, etc.), use loops:

  for (i = 0; i < G_N_ELEMENTS (abi->cached_modules); i++)
    g_list_free_full (abi->cached_modules[i].cached, g_object_unref);

  and when we're just dealing with one flavour of graphics modules, use an index, like:

        abi->cached_modules[SRT_GLX_GRAPHICS_MODULE] =
            _srt_list_glx_icds (self->env, self->helpers_path,
                                multiarch_tuple);

  Similarly, _srt_list_vdpau_drivers() and friends could become _srt_list_graphics_modules (SrtGraphicsModule, gchar **, const char *, const char *).

  I don't think this is a merge blocker, but I do think it's something we should do as a follow-up.