st/dri: Move dri2_format_mapping table and it's accessors from dri2.c to dri_helpers.c

8af1990a exposed dri2_get_mapping_by_fourcc() in dri_helpers.h, so it could be used by dri_get_egl_image(), but didn't move it. This breaks the build in the with_dri=false case (e.g. when building for a target which doesn't have libdrm, so swrast is only dri driver built)