targets/dri-swrast: convert to gallium megadrivers :)

targets/dri-swrast: convert to gallium megadrivers :)

Export the approapriate new symbol, and keep backwards compat
via the megadriver_stub helper library.

Our next step would be to unify dri/drm and dri/sw, leading to
a complete megadrivers solution, and having a single library
that provides dri across all targets.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Rob Clark <robclark@freedesktop.org>
Tested-by: Thomas Helland <thomashelland90 at gmail.com>
Acked-by: Tom Stellard <thomas.stellard@amd.com>