st/dri: simplify dri_get_egl_image by reusing dri2_format_table
this makes dri2_get_mapping_by_fourcc accessible from dri_helpers.h and does a direct lookup on the fourcc id to match the pipe format
v2 (Ken): Allow map to be NULL, use img->texture->format.
Reviewed-by: Kenneth Graunke kenneth@whitecape.org