mesa: pass gl_format to _mesa_init_teximage_fields()

mesa: pass gl_format to _mesa_init_teximage_fields()

This should prevent the field going unset in the future.  See bug
http://bugs.freedesktop.org/show_bug.cgi?id=31544 for background.

Also remove unneeded calls to clear_teximage_fields().

Finally, call _mesa_set_fetch_functions() from the
_mesa_init_teximage_fields() function so callers have one less
thing to worry about.