egl/sl: split out swrast probe into separate function

egl/sl: split out swrast probe into separate function

Make the code a bit easier to read.

As a bonus point this makes it obvious that we forgot to call
_eglAddDevice() for the device - do so.

v2:
 - s/dpy/disp/ (Eric)
 - free(driver_name) on dri2_load_driver_swrast() failure (Eric)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de> (v1)
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>