st/dri: Allow separate dri-targets

st/dri: Allow separate dri-targets

With this commit we add a couple of DEFINES making the ST code
conditional, in a way that we can use it to gradually convert
the dri-targets from separate libraries into a single one.

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>